J Clin Monit Comput DOI 10.1007/s10877-013-9456-3

REVIEW PAPER

Factors affecting hemoglobin measurement Lauren Berkow

Received: 19 September 2012 / Accepted: 20 March 2013  Springer Science+Business Media New York 2013

Abstract A review of the literature shows that current ‘‘standard’’ laboratory measurements for hemoglobin are subject to numerous factors that affect both accuracy and reliability. In addition, total hemoglobin concentration measurements are subject to numerous factors that affect the ‘‘true’’ hemoglobin value. This article discusses both the physiologic factors that influence hemoglobin levels and the technical aspects and variability among the different measurement methodologies currently available. Keywords Hemoglobin concentration
CO-Oximetry
Hematology analyzer
Point-of-care
Noninvasive

1 Introduction Hemoglobin carries oxygen from the lungs and delivers it to vital organs throughout the human body. The hemoglobin protein was discovered in 1840 by Hunefeld [1] and was first isolated via X-ray crystallography by Max Perutz in 1959 [2]. Hoppe-Seyler [3] was the first scientist to identify the protein’s ability to form a bond with oxygen, and named the protein hemoglobin. Hemoglobin concentration is a function of the circulating red blood cells and the concentration of plasma and white blood cells [4]. Hemoglobin concentration is commonly used in clinical medicine to diagnose anemia, identify bleeding, and manage red blood cell transfusions. Hemoglobin electrophoresis can diagnose hemoglobinopathies such as sickle

L. Berkow (&) Department of Anesthesiology and Critical Care Medicine, Johns Hopkins School of Medicine, 600 N. Wolfe St., Meyer 8-134, Baltimore, MD 21287, USA e-mail: [email protected]

cell disease, thalassemia, carboxyhemoglobinemia, and methemoglobinemia. Originally, hemoglobin concentration was indicated by hematocrit measurement. A blood sample was spun in a tube and the percent of sample that was clearly red in color represented the percentage of red blood cells in the sample, or hematocrit [4]. Today, spun hematocrit has largely been replaced by hemoglobin measurement on laboratory devices. Historically, hemoglobin has been measured via intermittent arterial or venous blood sampling, or in some care areas, with point-of-care devices that use capillary blood samples. New continuous, noninvasive methods of measuring hemoglobin have recently been introduced into the clinical environment. The current literature reveals that the accuracy of these noninvasive, continuous methods can be variable compared to laboratory hemoglobin measures. Most clinicians interpret a laboratory measurement and assume it would not change significantly if consecutive samples were measured repeatedly on the same laboratory device or on different laboratory devices. But how accurate are the laboratory hemoglobin measurements being used today? A review of the literature shows that ‘‘standard’’ laboratory measurements are subject to numerous methodologic factors that affect both accuracy (how close the measurement is to the actual hemoglobin value) and precision (how repeatable the measurement is). According to the International Organization of Standardization, the definition of a laboratory error is ‘‘any defect from ordering tests to reporting results and appropriately interpreting and reacting to these [5].’’ The reported total laboratory error rates, which include preanalytic, analytic, and postanalytic stages of testing, vary between 0.1 and 9.3 % for all laboratory measurements [6]. Laboratory error is only one potential source of variability in reported hemoglobin values. Numerous physiologic, temporal, and methodologic

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factors can cause variability in hemoglobin values as well. This article discusses the physiologic factors that influence hemoglobin levels in the blood, the process to obtain a sample for measurement, and the technical aspects and variability among the different measurement methodologies currently available.

2 Physiologic factors Many physiologic factors can affect a total hemoglobin concentration value. The source of the blood sample, patient body position at the time of the blood draw, and the use of a tourniquet to retrieve the blood sample can influence the laboratory value. The site of the body at which the sample is taken can also affect results. Additionally, hemoglobin values appear to have diurnal variations (Table 1).

2.1 Source of blood sample Although the number of circulating red blood cells is not likely to change dramatically over a short period, the hemoglobin concentration in a given blood sample can vary as the amount of plasma volume changes and as pulsating blood moves throughout the vascular system. Therefore, the laboratory hemoglobin value can vary depending on whether the source of the blood sample is arterial, venous, or capillary blood. Daae et al. [7] compared venous and capillary blood samples in adults and found that both hemoglobin and hematocrit values were higher in the capillary blood samples (mean ?2.4 % in hemoglobin, mean ?3 % in hematocrit). Neufeld et al. [8] compared venous and capillary blood samples in adults and children using two different spectrophotometric methods [HemoCue (Hemocue Inc., Cypress, CA, USA), a point-of-care device, and Cell-Dyn (Abbott Diagnostics, Lake Forest, IL, USA), a bench-top hematology analyzer] and reported similar

results. Hemoglobin values were on average 0.3–0.5 g/dL higher in the capillary samples. Mokken et al. [9] compared hematologic parameters in venous and arterial blood from adults and found hematocrit values to be higher in venous blood samples than in arterial samples (0.45 ± 0.05 vs. 0.43 ± 0.04; p \ 0.001). Yang et al. [10] compared venous blood samples to both arterial and capillary blood samples and assessed inter-measurement variation between samples. Hemoglobin values were 1.8 % higher and hematocrit values were 3.1 % higher in venous blood than in the arterial blood samples. However, no significant differences were detected in hemoglobin or hematocrit between venous and capillary samples. 2.2 Tourniquet usage Tourniquets are routinely used to obtain venous blood for laboratory testing when an indwelling catheter is not in place. The use of a tourniquet leads to venous stasis, which can potentially influence laboratory values. A study by Junge et al. [11] in 1978 compared venous samples after tourniquet application and found that prolonged tourniquet times resulted in a 4–9 % increase in hemoglobin values. More recent studies also have found higher hemoglobin and hematocrit values with tourniquet use of 2 min or longer with values increasing from 3.0 to 6.2 % [12, 13], although tourniquet times of \30 s appear to have minimal effect [14]. 2.3 Body position In contrast to conditions when subjects are supine, gravity causes decreased plasma volume and venous pooling in the dependent areas of the body when subjects are standing. Standing therefore can result in hemoconcentration of blood and higher hemoglobin levels. Lundvall and Bjerkhoel [15] compared arterial and venous blood samples in healthy non-fasted adults and found that both arterial and venous hemoglobin values significantly increased (up to 9 % after 15 min of standing) when

Table 1 Physiologic factors that affect hemoglobin measurement Physiologic factor Source of sample

Causes of variation a) Capillary blood has higher Hb than venous blood [7, 8] b) Venous blood has higher Hb than arterial blood [9, 10]

Tourniquet use

Tourniquet use longer than 30 s increases hemoglobin value [12, 61]

Body position

Hb is higher in blood samples from standing subjects than in samples from sitting or supine subjects [15–17]

Diurnal variation

Hb tends to be higher in morning and to decrease throughout the day [18, 19]

Site of blood sample

Hb may vary from right to left hand [21] and from finger to finger [22] Hb may differ between small and large vessels [23]

Hb Hemoglobin

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subjects moved from supine to standing. More recent studies have found similar results. Ahlgrim et al. [16] found a 2.4–2.7 % decrease in venous hemoglobin values when athletes moved from a standing to a seated position. Goldner et al. [17] found a 4 % change in hematocrit with changes in posture and called this phenomenon postural pseudoanemia.

3 Measurement methodology

2.4 Diurnal and day-to-day variation

3.1 Types of measurement devices

Both large epidemiologic studies and studies in which multiple samples from the same patient have been measured throughout the day have found that hemoglobin values tend to be higher in the morning and decrease throughout the day, [18] with a mean difference of 0.35 g/dL reported in one study [19]. Schumacher et al. [19], who studied hemoglobin values in male athletes, also found that hemoglobin values increased after exercise by a mean of 0.46 g/dL, (p \ 0.001) most likely due to a decrease in plasma volume. Looker et al. [20] found that hemoglobin values in both men and women vary from day to day and suggested that at least three repeated samples were necessary to obtain an accurate sample that is not biased by within-person variation. Within person variation was 0.28 g/dL in men and 0.23 g/dL in women. Morris et al. [21] also found significant within-person variability in hemoglobin values measured on four consecutive days, with a coefficient of variation of 7.0 %.

3.1.1 Cyanmethemoglobin assay

2.5 Site of blood sampling Hemoglobin values have been shown to vary within an individual depending on the site at which the blood sample is drawn. Boulton et al. [22] studied hemoglobin values tested via HemoCue from finger capillary samples of blood donors and found significantly different values in samples drawn from the same individual at the same time but from different fingers. The coefficient of variation ranged from 3 to 7 %. It is unclear whether the differences were due to true variation within the individual or device measurement variability. Hemoglobin values also have been shown to vary depending on whether the sample is taken from the left or right hand. Morris et al. [21] found a 6.3 % difference between capillary blood samples drawn from the left and right hand in the same individual. Differences in hemoglobin concentration are also found when comparing blood from small and large vessels. In an early study, Ebert and Stead [23] found hemoglobin concentration from small vessels to be 0.8–1.8 g/dL lower than hemoglobin in blood from a vein.

Hemoglobin and hematocrit can be measured by a variety of methodologies. None of the available devices is perfect and all have inherent variation, both between different types of devices and between devices that use the same technique.

The cyanmethemoglobin assay (HiCN) is considered the gold standard for measurement of hemoglobin levels in the blood [24]. The assay is performed by mixing the blood sample with a cyanide-containing reagent that converts the hemoglobin molecule into cyanmethemoglobin. The cyanmethemoglobin is then measured by photospectrometry. The photometers used by laboratories are calibrated frequently by using a cyanmethemoglobin standard approved and checked by the International Council for Standardisation in Haematology (ICSH). The ICSH also publishes recommendations for how this assay should be performed [25]. Although considered the gold standard, this assay is technique-dependent, time- and labor-intensive, and expensive, and it requires a large blood sample. Therefore, it is not practical for use in the clinical setting, where laboratory results are needed quickly for decision making [26]. The United States Food and Drug Administration (FDA) requires use of the HiCN assay for hemoglobin measurement device submissions, but this method is rarely used as the reference in clinical studies on hemoglobin measurement accuracy. Biases and standard deviations (SD) from some studies that used the HiCN method as the reference for hemoglobin measurement accuracy are shown in Table 2. 3.1.2 Hematology analyzer method The hematology analyzer counts hemoglobin proteins by detecting changes in conductance as cells suspended in a low concentration electrolyte solution pass through a small aperture [27, 28]. The volume of electrolyte displaced by the particle passing through the aperture causes a short-term change in the impedance across the aperture which is measured as a voltage or current pulse. Characteristics of the pulse are used to calculate the number and volume of particles. Hematocrit is calculated as the product of the mean cell volume and the red blood cell count, both of which are directly measured by the analyzer [29]. This method can also measure other values such as white blood cell volume and platelet count. One study reported a bias ± SD of 0.3 ± 0.2 g/dL

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J Clin Monit Comput Table 2 Accuracy of hemoglobin measurement methods compared to the gold standard, HiCN Method (model, brand) Coulter counter (M2000, Sysmex) CO-Oximetry (ABL-62, Radiometer) Spectrophotometric point of care (HemoCue)

Reported bias ± SD compared to HiCN (g/dL)

Study population

0.26 ± 0.18

50 postoperative patients [30]

-0.19 ± 0.28

50 postoperative patients [30]

0.17 ± 0.55

50 postoperative patients [30]

-1.2 ± 1.1

100 healthy children [37]

0.13 ± 0.26

398 healthy subjects [62]

HiCN Cyanmethemoglobin assay

between hemoglobin values determined by a Coulter Counter and the HiCN assay [30]. 3.1.3 Laboratory CO-Oximetry Laboratory CO-Oximetry, or hemoximetry, measures hemoglobin and hemoglobin oxygen saturation. Using a set of fixed wavelengths of light and the principles of the Beer–Lambert law,1 CO-Oximeters measure the intensity of light passing through a hemoglobin suspension to determine the concentration. CO-Oximeters are also used to calibrate pulse oximetry devices based on their ability to accurately measure oxygen saturation of hemoglobin. COOximetry was named because of its ability to measure carboxyhemoglobin and oxygen saturation. The accuracy of a CO-Oximeter is determined by the accuracy of the reference standard and the light source used [31]. The number of hemoglobin derivatives detected by the CO-Oximeter is determined by the number of fixed wavelengths it is able to detect. Most CO-Oximeters used clinically detect over 100 wavelengths of light, providing the ability to discriminate between hemoglobin derivatives such as methemoglobin and hemoglobin F [32]. Gehring et al. [26] measured the error among five different CO-Oximeter devices. Samples were tested on two identical devices from each manufacturer, and differences in hemoglobin values up to 1.2 g/dL were detected between the pairs. When hemoglobin values from six different CO-Oximeters were compared to that determined by a Coulter Counter, Patel et al. [33] found biases ± SD ranging from 0.0 ± 0.2 to 1.4 ± 0.4 g/dL. Differences of over 1 g/dL could influence the decision to transfuse a patient for anemia, and so may be clinically important.

1

Beer-Lambert law is the linear relationship between absorbance and concentration of an absorbing species and is typically written as: A = a(k) 9 b 9 c where A is the measured absorbance, a(k) is a wavelengthdependent absorptivity coefficient, b is the path length, and c is the analyte concentration.

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3.1.4 Conductometric point-of-care devices (iStat) The conductometric method uses conductivity to calculate hematocrit. Samples of arterial, venous or capillary whole blood are placed into biosensor cartridges that contain reference electrodes and specific sensors for the analyte to be measured. The devices measure conductivity, which is inversely related to hematocrit. Hemoglobin levels can be calculated from the measured hematocrit level. Results from the i-STAT device (Abbott Point of care, Princeton, NY) can be affected by elevated white blood cell values, high lipid levels, and low total protein levels [34]. Hopfer et al. [35] also found the i-STAT device to be less accurate than a hematology analyzer at low hemoglobin levels with sample discrepancies up to 2 g/dL. While the compactness and the low volume of blood required by this type of device is desirable in a diagnostic tool, a bias of up to 2 g/dL could affect clinical management of patients. For this reason, ‘‘borderline measurements’’ should be confirmed with another measurement methodology. 3.1.5 Spectrophotometric point-of-care devices (HemoCue) The HemoCue system uses spectrophotometry and singleuse cuvettes that draw blood by capillary action. In previous versions of the technology, reagents in the cuvette lysed the red cell membrane to convert the hemoglobin into azidemethemoglobin, which was then measured photometrically. The currently available microcuvettes do not contain reagents. The HemoCue system is a point-of-care testing device, meaning it can be used by non-laboratorytrained medical professionals at the site of care. It is fast and inexpensive and requires only small amounts of blood. One disadvantage of the HemoCue system is that it has large inter-operator variability [8]. Although the manufacturer recommends using a capillary sample obtained from the finger by a lancing device, the results are less accurate when capillary blood samples are used than when arterial or venous blood is used. The introduction of air bubbles or lack of sample mixing before analysis can also cause errors [36]. Studies show that HemoCue values can

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vary from HiCN values by 1.2 g/dL [37] and from Coulter values by 0.3 g/dL [8] when used to estimate hemoglobin in children.

3.1.6 Pulse CO-Oximetry for noninvasive hemoglobin measurement One commercially available technology has received United Stated FDA 510(k) clearance for noninvasive hemoglobin measurements [38]. Noninvasive pulse COOximetry technology is available in multiple devices that can be used for continuous (Radical-7, Rad-87, Rad-57) and spot-check (Pronto-7, Pronto) measurements (Masimo Corporation, Irvine, CA). The noninvasive sensor (adhesive for continuous monitoring, reusable for spot checks) detects seven or more wavelengths of light and then, via complex algorithms and filters calculates a hemoglobin value. Noninvasive devices offer the advantages of not requiring a blood sample and allowing continuous assessment of hemoglobin between intermittent and invasive blood sampling. The accuracy of noninvasive hemoglobin measurement compared to invasive methods, however, remains controversial. Some studies have found moderate to large biases and limits of agreement compared to invasive methods [39, 40] whereas other studies have shown biases and limits of agreement within the range of invasive methods [41–43]. The difference in performance may be due to multiple factors including different versions of the device and sensors used for the noninvasive measurements, differences in patient populations between studies, and whether appropriate study methodology for reporting diagnostic accuracy were followed [44]. Some studies show that accuracy of these noninvasive devices diminishes with poor peripheral perfusion, [39] as may occur in patients with vasoconstriction due to major blood loss or patients receiving vasopressors [45], but other studies did not [43, 46]. In addition, the manufacturer recommends covering the sensor to reduce interference from other optical sensors in close proximity. Rather than replace invasive measurements, noninvasive monitoring may be a useful adjunct to laboratory measurements by providing continuous data, as the percent change in hemoglobin values has been shown to have greater prognostic value than nadir hemoglobin levels during cardiac surgery [47]. Another noninvasive measurement device, the NBM200MP (Orsense, Nes Ziona, Israel), measures hemoglobin via a ring-shaped sensor that intermittently squeezes the finger, similar to a blood pressure measurement, and then measures hemoglobin using light absorption. This technology is intermittent and has not been cleared for use by the FDA in the United States as of this writing.

3.2 Inter-device variation Hemoglobin values can vary depending on the method of measurement used. Several studies have compared laboratory methods and demonstrated that hemoglobin measurements from the same blood sample can differ significantly. Rivas Chirino et al. [48] compared hemoglobin measurements in patients undergoing liver transplantation using both the Coulter method and arterial blood gas analyzers. Hemoglobin values were consistently higher when measured by the blood gas analyzer (0.3–1.0 g/dL). Bourner et al. [49] compared four hematology analyzers from different companies and found significantly different false-positive rates for flagging (automatic trigger to review a slide) between the analyzers, with some as high as 15 %. In addition, the four analyzers did not perform equally in their ability to handle aged blood samples. Patel et al. [33] compared nine different analyzers and found similar variations in hemoglobin values. Recent studies that have evaluated noninvasive hemoglobin monitoring devices also have shown differences between laboratory hemoglobin values and noninvasive hemoglobin measurements recorded at the same time as the laboratory draw. Two recent studies that evaluated the Masimo Radical-7 monitor in patients undergoing spine surgery revealed differences from 0.1 g/dL [42] to at least 1.5 g/dL [39] between the noninvasive hemoglobin values and hemoglobin levels measured via laboratory COOximetry. Lamhaut et al. [46] compared noninvasive hemoglobin monitoring via spectrophotometry to HemoCue point-of-care testing and laboratory measurement via a Coulter Counter and found that the noninvasive monitor gave lower hemoglobin readings than did the HemoCue and laboratory devices, as well as a higher percentage of outlier values (46 vs. 16 %). In a study conducted in the intensive care unit (ICU) on 33 patients presenting with severe gastrointestinal bleeding, Coquin et al. [45] found a significant correlation between SpHb and venous hemoglobin by a hematology analyzer, but a large bias and standard deviation (1.0 ± 1.9 g/dL) when using various earlier versions of the SpHb sensor. Although finger perfusion was not correlated with SpHb accuracy, there was a higher percentage of unavailable SpHb measurements (no-reads) in patients receiving norepinephrine. The primary end point of Coquin’s study was the percentage of inaccurate measurements compared to a reference value (hemoglobin measured via a venous sample), and the percentage of inaccurate measurements were significantly higher (56 %) with SpHb compared to capillary samples (15 %). Capillary HemoCue measurement on the same subjects had a bias ± SD of 0.4 ± 1.0 g/dL. When the difference in consecutive hemoglobin values from SpHb or HemoCue was compared to the difference in consecutive

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hemoglobin values of the hematology analyzer, a significant correlation was found for both methods (p \ 0.001). The coefficients of determination (R2) were 0.32 for SpHb and 0.43 for HemoCue. The authors concluded that noninvasive hemoglobin measurement had unacceptable accuracy for guiding transfusions and the unavailability of measurements in patients receiving norepinephrine represented another significant limitation of the technology. In another study conducted in the ICU on 62 patients, Frasca et al. [43] compared the agreement of hemoglobin values from noninvasive hemoglobin, benchtop CO-Oximetry, and HemoCue point-of-care testing to a laboratory hematology analyzer used as the reference device. Arterial blood samples (471) were used for all invasive measurements. Compared to the laboratory hematology analyzer, the bias and SD was 0.0 ± 1 g/dL for noninvasive hemoglobin, 0.9 ± 0.6 g/dL for the benchtop CO-Oximeter, and 0.3 ± 1.3 g/dL for the point-of-care device. Contrary to the results of Coquin et al. [45], in this study, noninvasive hemoglobin measurements had as good or better agreement with the reference measurements as did measurements obtained invasively. Further, when the coefficients of determination (R2) were calculated for the difference in consecutive hemoglobin values from each test method and compared to the difference in consecutive hemoglobin values of the reference method, noninvasive hemoglobin had better trending (R2 = 0.41) than did the CO-Oximeter (R2 = -0.36) or the point-of-care device (R2 = 0.15). It is unclear what variables contributed to the differences in the results between these two studies, be it the different versions of the SpHb sensor used, differences in the patient populations, or other factors. When Coquin et al. [50] repeated their study using the noninvasive Orsense device (instead of SpHb) and capillary Hemocue compared to venous hemoglobin by a laboratory hematology analyzer, results were similar. Bias and standard deviation were0.4 ± 2.0 g/dL for Orsense and 0.8 ± 1.2 g/dL for capillary HemoCue. Further investigation on how current versions of these noninvasive technologies perform in patients with active bleeding is warranted. A recent editorial published in connection with the Coquin study suggests that while noninvasive measurement is the best approach to monitor trend in Hb concentration or if blood collection is a concern,…invasive Hb measurement should be the first choice if a better accuracy is important or if reading SpHb is problematic (shock, hypothermia) [51]. 3.3 Intra-device variation Hemoglobin measurements can also vary within the same device. This variation can result from patient-related hemoglobin variability but also from variation among different machines or how they are calibrated. The time

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between when the sample is drawn and when it is analyzed, or the age of the sample, has also been shown to affect results [49]. Laboratory measurement devices are often compared to a reference standard, but this standard is also subject to error. Bland and Altman [52] showed that both reference and test devices are subject to error and that these sources of error must be taken into consideration when assessing the accuracy of methodologies. Using blood drawn from the fingertip and arterial and venous sources, Yang et al. [10] tested the same sample three times on the same analyzer device and found modest inter-measurement variation with coefficients of variation of 2.45 % for fingertip, 1.46 % for venous and 1.30 % for arterial blood. Gehring et al. [26] studied the variation in values obtained from two identical hemoximeters produced by five different manufacturers and found variations up to 1.2 g/dL SD in hemoglobin measurements between devices. Intra-device variation therefore differs by analyzer and is an important factor is assessing analyzer quality. Assessment of the literature regarding the agreement of hemoglobin measurements from different non-gold-standard methodologies shows a wide range of biases and standard deviations between devices that may be due to different patient populations, differences in specimen handling, inter- and intra-device variation, or all of the above (Table 3).

4 Preanalytic error Pre-analytic errors are those that are introduced by any means, prior to analysis of the sample. These include errors introduced during specimen collection, transport, and processing of the sample before analysis. Preanalytic errors can affect variation and reliability of laboratory testing and ultimately decisions made about clinical care. With the radical improvement in technology over the past few decades and reduction in analytic errors, preanalytic errors may now play a larger role in variation and reliability of laboratory testing. Lack of standardized protocols for specimen handling and collection may also contribute to preanalytic errors [6]. Lippi et al. [53] tracked preanalytic errors over a 1-year period and found a higher rate of errors in inpatient samples (0.82 %) than in outpatient samples (0.37 %). Another survey of outpatient laboratory samples found that the most common causes of preanalytic error were hemolysis (18 %), insufficient sample quantity (16 %), and sample clotting (13 %) [54]. Common sources of preanalytic error that can prevent obtaining accurate hemoglobin measurements include: • •

Incorrect/missing identification on sample [54, 55]. Improper container [55, 56].

J Clin Monit Comput Table 3 Agreement of non-gold-standard hemoglobin measurement methods Method 1 (model, brand)

Method 2 (model, brand)

Bias ± SD (g/dL)

Study population

Coulter counter (XT-2000i, Sysmex)

CO-Oximeter (RapidPoint, Siemens)

0.9 ± 0.6

62 ICU patients [43]

(XT-2000i, Sysmex)

Spectrophotometric point of care (HemoCue)

0.3 ± 1.3

62 ICU patients [43]

(SE9500, Sysmex)

Spectrophotometric point of care (HemoCue)

-0.06 ± 0.87

94 patients with GI bleeding [63]

(XE2100, Sysmex)

Spectrophotometric point of care (HemoCue)

0.2 ± 0.76

198 ICU patients [64]

(STKS, Sysmex)

Spectrophotometric point of care (HemoCue)

0.63 ± 1.3

132 dialysis patients[36]

(XE-2100, Sysmex)

Conductometric point of care (i-Stat)

1.17 ± 0.76

60 ICU patients [65]

(XT-2000i, Sysmex)

Noninvasive SpHb (Radical-7, Masimo)

0.0 ± 1.0

62 ICU patients [43]

(SP-1000i, Sysmex)

Noninvasive SpHb (Radical-7, Masimo)

-0.02 ± 1.4

44 urologic surgery patients [46]

(ABL800, Radiometer)

Noninvasive SpHb (Radical-7, Masimo)

-0.1 ± 1.0

29 spine surgery patients [42]

(ABL820, Radiometer)

Noninvasive SpHb (Radical-7, Masimo)

-0.15 ± 0.92

20 hemodilution subjects [41]

(Beckman Coulter)

Noninvasive SpHb (Radical-7, Masimo)

0.26 ± 1.8

20 spine surgery patients [39]

CO-Oximeter

Comparison of two devices of same model/manufacturer CO-Oximeter

12 healthy volunteers [26]

(STP-CX, Nova Biomedical) (682 CO-Oximeter, Instrumentation Laboratory)

-0.77 ± 0.29 0.38 ± 1.18

(ABL735, Radiometer)

0.00 ± 0.06

ICU Intensive care unit, GI gastrointestinal, SpHb spectrophotometric hemoglobin

• • • • • •

Insufficient sample volume [56]. Hemolysis [54, 56]. Clotting [56]. Contamination [56]. Lag time before sample analysis [49, 57]. Prolonged tourniquet use/venous stasis [12, 55].

5 Discussion Hemoglobin measurement plays an important role in the clinical evaluation of patients. Hemoglobin and hematocrit measurements are the primary methods used to diagnose anemia and guide blood transfusions. Currently, a variety of clinical methods exist to quantify the amount of hemoglobin protein in the blood. Nearly all of these methods provide single values at a specific point in time, and all but two require a blood draw, an invasive procedure that must be performed by trained medical personnel. Often, hemoglobin measurement requires a clinical laboratory, and

laboratory expenditures account for approximately 5 % of total hospital costs in the United States [56, 58]. The apparent variability of hemoglobin concentration within an individual depends on multiple variables, such as the source of the blood sample, body position, time of day, and site of the blood draw. When using hemoglobin measurements to guide clinical care of an individual patient, medical providers should recognize these sources of variation. In addition, hemoglobin measurements may vary depending on the type of device used to generate the result. Identical devices from the same manufacturer demonstrate variation in performance and calibration; a blood sample run on one machine in a clinical or laboratory setting might generate a different value than one run on another machine of the same brand in the same setting. When important decisions are made regarding matters such as the need for blood transfusion, clinicians should understand the variability that can exist with hemoglobin measurements and take into account that all laboratory and reference devices are subject to error. In addition to cost, administration of

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blood products poses risks to the patient that must be weighed against the potential benefit of transfusion. Newer technologies, such as SpHb monitoring with Pulse COOximetry, now provide the ability to measure hemoglobin continuously and noninvasively. This could potentially remove or reduce the need for a blood sample as well as reduce some sources of error and variability, but further studies are required to fully elucidate the clinical value of these non-invasive devices.

6 Conclusion Hemoglobin measurement is a common and critical test, frequently ordered by healthcare providers to detect anemia and guide blood management. The ability to track a patient’s hemoglobin status is becoming increasingly important, especially in critical care, as the clinical evidence builds regarding the dangers of both sustained anemia [59] and over-transfusion [60]. Despite the ubiquity of hemoglobin testing across nearly all areas of healthcare, the numerous sources of potential variation in hemoglobin measurement may not be fully appreciated by those who use these values for clinical decisions. When laboratory data is used for clinical decision making, it is important that the clinician be aware of the variability of laboratory values. Preanalytic sources of variability, including patient physiology, time of sample collection, tourniquet use, sample handling, and analytic factors such as the measurement methodology, all contribute to hemoglobin measurement variability. To reduce variability in individual patients, clinicians should try to maintain consistency of these factors between measurements, for example by drawing blood from the same source and with the patient in the same position. The accuracy of emerging hemoglobin measurement technologies needs to be evaluated in light of all these potential sources of variation, which affect both the test and the reference measurements. Conflict of interest The author is a paid consultant and member of the Scientific Advisory Board for Masimo Corporation (Irvine, CA).

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