A Comparison of Four Bedside Methods of Hemoglobin Assessment During Cardiac Surgery Stephen E. McNulty, DO, Marc Torjman, EdM, Wlodzimierz Grodecki, MD, Alex Marr, CBNA,

and Hugh Schieren, rhD

Jefferson Medical College, Thomas Jefferson University, Philadelphia, Pennsylvania

The purpose of this study was to compare the accuracy of conductivity, adjusted conductivity, photometric, and centrifugation methods of measuring or estimating hemoglobin (I%) with Coulter measured HB as the ref- erence. These bedside methods were studied in 25 car- diac surgery patients during euvolemia and hemodilu- tion and after salvaged autologous red blood cell transfusion. In viva patient blood samples were ob- tained before induction, at the start of cardiopulmonary bypass (CPB), after CPB, and after blood transfusion. In 10 patients, blood was sampled in vitvo from units of processed blood. Hb values were determined using conductivity by Stat-Crit@, adjusted conductivity by Nova Stat Profile 9, bedside photometry by HemoCue@, and centrifugation methods. The calculated bias values of Coulter test method Hb (mean -C SD) for in viva pa- tient blood samples (n = 90) were: Stat-Crit@ = 0.6 2 0.8

g/dL; Nova Stat Profile 9 = -0.7 ? 0.4 g/dL; HemoCue@ = -0.1 -C 0.2 g/dL; and centrifuge = 0.1 + 0.5 g/dL (P < 0.0001). Hb bias values (g/dL) for in vitro samples (n = 10) obtained from processed blood were Stat-CriP = 5.1 +- 0.6; Nova Stat Profile 9 = 3.0 ? 0.6; HemoCue@ = 0.4 t 0.4; and centrifuge = 0.6 5 0.3 (P < 0.0001). Hb assessment by different test methods may be significantly affected during hemodilution and after blood transfusion. In vitro conditions exaggerated the inaccuracy of conductivity and adjusted conductivity Hb estimates. The rank order of closest approximation to the Coulter measurement for all in vivo blood sam- ples was provided by bedside photometry, followed by centrifugation, adjusted conductivity, and uncorrected conductivity methods.

(Anesth Analg 1995;81:1197-202)

B edside instruments that measure or estimate whole blood hemoglobin 0%) in the operating room may be affected by hemodilution, red blood

cell damage associated with cardiopulmonary bypass (CPB), and red blood cell transfusion. The conductivity method has been demonstrated to be accurate under normal conditions (l-4). However, this method becomes inaccurate when serum electrolytes and/or protein con- centrations deviate from normal, as during hemodilu- tion (5). An adjusted conductivity Hb estimate can be obtained by correcting for changes in non-red-cell con- stituents. Consequently, multifunction instruments that also measure electrolytes may correct the conductivity measurements, usually with an algorithm based on the sodium concentration. Photometry is another method used to quantify the Hb content of whole blood. This

technique was demonstrated to be accurate in nonopera- tive clinical settings (6-10) and with in vi&o hemodilu- tion (ll), but has not been evaluated with in viva he- modilution and transfusion. Centrifugation estimations for Hb are sensitive to factors that alter the Hb content of red blood cells or result in red cell lysis, such as CPB. The relative accuracy of whole blood centrifugation has been compared to uncorrected conductivity (5) but not to bedside photometry or adjusted conductivity techniques during the clinical conditions associated with CPB, he- modilution, and blood transfusion.

This study compares bedside estimations or meas- urements of Hb with the Coulter counter using con- ductivity, adjusted conductivity, photometric, and centrifugation techniques. The purpose of this study was to evaluate the relative accuracy of four different methods of assessing Hb intraoperatively, under clin- ical conditions that included euvolemia, hemodilu- tion, and red blood cell transfusion.

Accepted for publication June 13, 1995. Address correspondence and reprint requests to Stephen E.

McNulty, DO, Department of Anesthesiology, Thomas Jefferson University Hospital, 111 South 11th St., Suite G-6460, Philadelphia, PA 19107-5092.

Methods

After approval by our institutional review board, 25 patients undergoing cardiac surgical procedures, gave

01995 by the International Anesthesia Research Society 0003-2999/95/$5.00 Anesth Analg 1995;81:1197-202 1197

11% CARDIOVASCULAR ANESTHESIA MCNULTY ET AL. ANESTH ANALG BEDSIDE HEMOGLOBIN MEASUREMENTS 1995;81:1197-202

written informed consent to participate in this study. There were no exclusion criteria for this study.

Four test methods were evaluated: conductivity by Stat-Crit@ (Wampole Laboratories, Cranbury, NJ); ad- justed conductivity by Nova Stat Profile 9 (Nova Bio- medical, Waltham, MA); photometry by HemoCue@ (HemoCue, Inc., Mission Viejo, CA); and centrifuga- tion. Centrifugation hematocrit measurements were performed in triplicate and the average value was divided by three to convert to Hb. The method of conversion of centrifuge hematocrit to Hb corre- sponded to methods incorporated into conductivity and adjusted conductivity devices. All other Hb tests were performed sequentially using a single meas- urement by a single operator. The control Hb was measured using the Coulter method, Coulter@ STKR 1 (Coulter Electronics, Inc., Hialeah, FL) by our institu- tion’s clinical laboratory. Blood sodium and chloride were measured using the Nova Stat Profile 9 and serum total protein was determined by laboratory analysis for each blood sample.

Patient blood samples (15 mL) were obtained before induction of anesthesia (Time l), after the start of CPB (Time 2), after termination of CPB (Time 3), and after transfusion of all available autologous blood (Time 4). Autologous blood transfusions before Time 4 con- sisted of cell-saved red blood cells in 10 patients and residual bagged pump blood in 15 patients. Scav- enged whole blood was processed using a Dideco BT-795-A Cell Saver and 1000 mL of normal saline for concentrating and washing red blood cells.

All test Hb measurements were performed immedi- ately after sampling from a single 20-mL syringe of blood that was mixed continuously. Blood was ob- tained from an indwelling radial artery catheter at Time 1 (n = 25) and at Time 4 (n = 25). At Times 2 (n = 25) and 3 (n = 15) blood was obtained from a stopcock connected to a continuous purge line from the arterial filter of the CPB circuit. Patient blood samples from Times 1 to 4 (n = 90) were considered as in vivo. Standardized methods were used for all blood sampling. All radial artery blood samples were ob- tained after first withdrawing 10 mL of blood, due to the presence of crystalloid in the high-pressure tubing to the transducer. All CPB blood samples were ob- tained after withdrawing and reinjecting 3 mL of blood.

To compare the different methods under in vitro conditions, 10 blood samples (15 mL) were obtained from bags of cell-saved blood. Tests were performed identically to those for the in viva blood samples. Blood samples obtained from the bags of processed blood were considered in vitro. Sodium and chloride were measured using the Nova Stat Profile 9 and serum total protein was determined by laboratory analysis for each of these blood samples as well.

The Stat-CriP, Nova Stat Profile 9, and HemoCue@ systems were confirmed to be operating within estab- lished limits by using the manufacturers’ calibration and maintenance protocols. All study measurements were performed within 3 h of quality control testing. The Coulter counter was calibrated against a reference standard (5C Coulter” Cell Control). Precision analy- sis for Hb measurement by the Coulter method had a mean difference of 0.02 + 0.08 mg/dL using a refer- ence standard.

All patients received isotonic intravenous fluids of Plasmalyte@ balanced salt solution (Baxter Healthcare Corp., Deerfield, IL) and normal saline. The CPB prime was 1600 mL Plasmalyte@ and 500 mL Hespan@ (McGaw Inc., Irvine, CA) with 12.5 g mannitol, 44 mEq sodium bicarbonate, and 10,000 U heparin.

Bias was calculated for each test method by Coulter Hb - test method Hb. The bias values for conductiv- ity, adjusted conductivity, photometric, and centrifu- gation test methods were compared using a factorial analysis of variance (ANOVA). Between-group differ- ences were subsequently determined using Fisher’s protected least significant difference (PLSD).

The changes in bias resulting from hemodilution and blood transfusion for in vivo blood samples at Times 1,2,3, and 4 were compared using ANOVA for repeated measures. Iul vitro bias values were compared to in vivo bias values with the Kruskal-Wallis test.

Percentage differences from the Coulter I-lb were cal- culated by [(Coulter Hb - test method Hb)/Coulter Hb] X 100. Absolute values of the percentage differences between the different test methods were compared using ANOVA.

The changes in total protein, chloride, and sodium resulting from hemodilution and blood transfusion for in vivo blood samples at Times 1, 2, 3, and 4 were compared using ANOVA for repeated measures. In vitro blood chemistries were compared to in vivo blood chemistries with the Kruskal-Wallis test.

Correlations between the Coulter Hb and paired test method values were performed by linear regression analysis.

Coulter Hb served as reference in the figure plots. All statistical analyses were performed using the Stat View IITM statistical package (Abacus Concepts, Inc., Berkeley, CA). A P < 0.05 was considered to be sta- tistically significant. All results are reported as mean k SD.

Results

The range of Coulter Hb values for all sample times was 4.2-20.7 g/dL. The frequency distributions of cal- culated bias values (Coulter - test method Hb) were considered to be normal for the in vivo (n = 90) and in

ANESTH ANALG CARDIOVASCULAR ANESTHESIA MCNULTY ET AL 1199 1995;81:1197-202 BEDSIDE HEMOGLOBIN MEASUREMENTS

2 4 6 a IO 12 14 16

Coulter Hemoglobin (g/dL)

-3 2 4 6 8 10 12 14 16

Coulter Hemoglobin (g/dL)

Figure 1. Plot of the Coulter-Stat-Crit@ bias for all WI VIXJ blood Figure 2. Plot of the Coulter-Nova bias for all in viva blood samples samples versus the corresponding Coulter hemoglobin. The outer versus the corresponding Coulter hemoglobin. The outer lines in- lines mdlcate the limits of 2 SD (1 6 g/dL) above and below the mean dicate the limits of 2 SD (0.8 g/dL) above and below the mean of of 0.6 g/dL. ~0.7 g/dL.

vitro (n = 10) blood samples. The frequency distribu- tions of in vivo and in vitro data combined was non- parametric. The in vivo Hb bias values for each of the test methods were: Stat-Crit@ = 0.6 ? 0.8 g/dL (Figure 1); Nova Stat Profile 9 = ~0.7 -t 0.4 g/dL (Figure 2); HemoCue@ = -0.1 +- 0.2 g/dL (Figure 3); and centri- fuge = 0.1 ? 0.5 g/dL (Figure 4). These values were significantly different from each other (P < 0.0001, ANOVA, Fisher’s PLSD). Hb bias values for in vitro samples (n = 10) obtained from processed blood were Stat-CriP = 5.1 -+ 0.6 g/dL; Nova Stat Profile 9 = 3.0 ? 0.6 g/dL; HemoCue @ = 0.4 t 0.4 g/dL; and centri- fuge = 0.6 +- 0.3 g/dL.

Due to the wide range of Hbs in this study, the percentage difference from the Coulter Hb was deter- mined for each test method. The absolute value (mean 2 SD) percentage differences for each instrument were: Stat-CriP = 11.7% t 8.7%, Nova Stat Profile 9 = 9.0% t 5.5%; HemoCue@ = 1.6% ? 1.3%; and centri- fuge = 4.8% ? 4.7%. These differences were signifi- cant by ANOVA (P < 0.0001) and, as determined from post hoc tests, each was significantly different from the other methods (P < 0.002, Fisher’s PLSD). Of 90 in vivo samples, the number with a > 10% difference from Coulter measured Hb were: Stat-Crit@ = 42; Nova Stat Profile 9 = 37; HemoCue@ = 0, and centrifuge = 11. The incidence of over- or underestimation of Coulter Hb when the Hb was < 8 g/dL is summarized in Table 3.

Hemodilution and red blood cell transfusion re- sulted in significant differences in calculated bias val- ues for Stat-Crit@, Nova Stat Profile 9, and HemoCue@ from Time 1 (before induction) to Times 2, 3, and 4 (Table 1). Comparison of in vivo bias values (Table 1) were significantly different from the in vitro values for

3-

-3 2 4 6 8 10 12 14 16

Coulter Hemoglobin (g/dL)

Figure 3. Plot of the Coulter-HemoCue@ difference (bias) for all in vim blood samples versus the corresponding Coulter hemoglobin. The outer lines indicate the limits of 2 SD (0.4 g/dL) above and below the mean bias of -0.1 g/dL.

the different test methods (P < 0.0001, Kruskal-Wallis test).

Hemodilution and red blood cell transfusion also resulted in significant differences in values for total protein, chloride, and sodium for the in vivo (Times 1 to 4, n = 90) and in vitro samples. Sodium and chloride were significantly higher, and the total protein was significantly lower, in the processed cell-saver sam- ples (Table 2) compared to the in vivo blood samples (P < 0.0001, Kruskal-Wallis test).

Linear regression analyses of Coulter Hb versus test-method Hb for all measurements yielded X2 val- ues of 0.938, 0.972, 0.996, and 0.960 for Stat-CriP,

1200 CARDIOVASCULAR ANESTHESIA MCNULTY ET AL. ANESTH ANALG BEDSIDE HEMOGLOBIN MEASUREMENTS 1995,81 1197-202

Table 1. Hemoglobin (g/dL) Bias for StatCrit @, Nova Stat Profile 9 (Nova), HemoCue@, and Centrifuge for In Vim (Times l-4) and In Vitro Measurements

Time

1 2 3 4 In vitro

Stat-CriP -0.3 + 0.5”” 0.9 -c 0.7 1.0 +- 0.7 0.9 i 0.8 5.0 ” 0.5”” Nova -1.0 k 0.5b,* -0.5 -c 0.4 -0.5 t 0.4 -0.7 i- 0.3 3.0 + 0.6’” HemoCue@ -0.2 i- 0.2- -0.0 2 0.1 -0.1 -+ 0.1 0.1 i 0.1 0.4 i 0.4’6 Centrifuge -0.1 + 0.7 0.2 -c 0.3 0.4 t 0.4”,g 0.0 i- 0.4 0.6 t 0.3’”

Values are mean i so Txne 1 = before mductmn of anesthesia, Time 2 = after start of cardiopulmonary bypass (CPB), Tune 3 = after termmatlon of CPB, Time 4 = after transfusion

of all available autologous blood; ANOVA = analysts of variance, Fxsher’s PLSD = Fisher’s protected least slgmficant difference, Bias = Coulter - test method hemoglobin.

‘I Stat-Crit” bias was significantly different at Time 1 compared to other times (F value = 23.6; P < 0.0001). ’ Nova bias was significantly different at Time 1 compared to other times (P value = 7.7; P < 0.0005, ANOVA, Fisher’s PLSD) ‘ HemoCue~ bias was sqq&cantly different at Tune i compared to other hmes (F value = 4 5; P < 0 01). ’ Centrifuge bias was slgmflcantly different at Tm~e 3 compared to other times (F value = 4.1, P < 0 02) ’ Palred 111 vztro measurements were slgmficantly different from each other with the exception of HemoCue@ vs centrifuge (P i 0 0001, ANOVA, Fisher’s

PLSD) fThe in zvtro measurements were slgmficantly different from measurement Times 1-4 (I’ < 0.0001, Kruskal-Walhs test). s ANOVA for repeated measures, djl = 3, df2 = 14, Fisher’s PLSD.

-3- 2 4 6 8 10 12 14 16

Coulter Hemoglobin (g/dL)

Figure 4. Plot of the Coulter-centrifuge bias for all lrl VKIO blood samples versus the corresponding Coulter hemoglobin. The outer lines indicate the limits of 2 SD (1 .O g/dL) above and below the mean of 0.1 g/dL.

Nova Stat Profile 9, HemoCue@, and centrifuge, respectively.

Discussion

This study evaluated the differences between various bedside methods for Hb estimation or determination compared to clinical laboratory measurements per- formed with Coulter counter instrumentation. Cardiac surgery patients were studied to achieve a wide range of Hb values under in viva conditions that included euvolemia, hemodilution, and blood transfusion. Of the four test methods used in this investigation, only the HemoCue@ measured Hb directly. Although the two conductivity methods provide a Hb reading, it is

calculated from a measured hematocrit then divided by three for conversion to Hb. The Coulter measures Hb and hematocrit independently. Good correlation (X2 = 0.995) and a low bias + SD (0.2 -+ 0.18 g/dL) between the Coulter Hb and the Coulter hematocrit converted to Hb provide some justification for using a fixed conversion ratio. Comparisons between the test methods were also made using the Coulter hematocrit as the reference standard; however, the results were not significantly different. Hb, rather than hematocrit, was chosen as a basis for comparing these devices since it is most relevant to the oxygen-carrying capac- ity of blood.

The uncorrected conductivity percentage difference from Coulter measured Hb was significantly greater than adjusted conductivity, photometric, and centrif- ugation methods. The observed differences in our study between Coulter and conductivity meas- urements were consistent with earlier findings (l- 5,12,13). Some of the previous studies reporting on the relative inaccuracy of conductivity measurements have used in vitro conditions in the evaluation. In the present study, blood processing with the cell salvag- ing device resulted in disproportionate increases in chloride and sodium, as well as a decrease in total protein, that were outside the range of electrolytes and protein encountered with the in viva blood samples.

Adjusted conductivity estimates of Hb were not as accurate as the centrifugation and bedside photomet- ric methods. The Nova Stat Profile 9 corrects the he- matocrit (Hct) electrical resistance measurement based on changes in sodium. The algorithm is Hct % = l- (l/Rc,,,) X 100, where I+,,, = RSample X ([Na+]/140)“2. Rcorr and RSample are the corrected and uncorrected blood sample resistances, respec- tively. This resul.ted in an overestimation of Hb com- pared to the Coulter measurement, possibly due to the

ANESTH ANALG 1995;81:1197-202

CARDIOVASCULAR ANESTHESIA MCNULTY ET AL. 1201 BEDSIDE HEMOGLOBIN MEASUREMENTS

Table 2. Hemoglobin (Hb), Total Protein, Chloride, and Sodium for In Vim (Times l-4) and In Vitro Measurements

Time

1 2 3 4 In vitro

Coulter Hb (g/dL) 11.4 t 1.4”J 7.3 -c 1.1 6.0 + 1.2 7.5 + 1.3 18.2 -c 1.2’ Total protein (mmol/L) 5.5 5 0.6bTf 3.4 + 0.4 2.9 t 0.8 3.3 5 0.6 0.4 -c 0.2’ Cl (mmol/L) 103 ? 5C,f 105 t 4 110 + 4 110 +- 4 134 k 4” Na (mmol/L) 139 5 6 138 -c 3 137 + 2d,f 140 ? 3 148 -c 2’

Time 1 = before induction of anesthesia; Time 2 = after start of cardiopulmonary bypass (CPB); Time 3 = after termination of CPB; Time 4 = after transfusion of all available autologous blood.

a Coulter Hb was significantly different at Time 1 compared to other times (F value = 95.6; P < 0.0001). b Total protein was significantly different at Time 1 compared to other times (F value = 72.8; P i 0.0001). ‘Chloride was significantly different at Time 1 compared to Times 2 and 3 (F value = 19.6; P i 0.0001). d Sodium was significantly different at Time 3 compared to Time 4 (F value = 2.8; P < 0.05) e In vitro measurements were all significantly different from measurement Times 1-4 (P < 0.0001; Kruskal-Wallis test). f Analysis of variance for repeated measures, dfl = 3, df2 = 14, Fisher’s protected least significant difference.

Table 3. Incidence of a 10% Over- or Underestimation of 49 Samples with a Coulter Measured Hemoglobin Values <8 g/dL

10% Within 10% Over 10% Under

Stat-CriP l/49 14/49 34/49 Nova Stat Profile 9 25/49 24/49 0 HemoCue@ 0 49/49 0 Centrifuge l/49 42/49 6/49

10% over = >lO% overestimation of Coulter hemoglobin by test method; within 10% = test method hemoglobin that was within 10% of the Coulter hemoglobin; 10% under = >lO% underestimation of Coulter hemoglobin by test method.

exclusion of total protein and chloride in the algorithm (5). It should be noted that the precision of the Nova Stat Profile 9 (SD of bias) was better than that of the centrifuge and Stat-Crit@ so that with a simple bias adjustment, precision of the percentage difference would be reduced to 6%. The precision of the Nova Stat Profile 9 measurements enables clinical manage- ment comparable to the HemoCue@ and centrifuge, as long as relative changes in Hb are considered and an absolute value for Hb is not used as a deciding factor for blood transfusion.

The agreement between bedside photometry and Coulter methods was similar to a previous test of this device in an in vitro hemodilution model (10). How- ever, the differences in accuracy between bedside pho- tometry and conductivity methods were less in the present study with in vivo blood samples than with those found in the previous in vitro study (10).

The centrifuge was nearly as accurate as the bedside photometry method at estimating Coulter Hb in this study. There was a tendency toward a slight underes- timation of Hb at low Hb levels. The potential for significant inaccuracy based on variations in red cell Hb content and hemolysis related to CPB was evident in only a small percentage of patients.

The expected percentage difference between several instruments measuring Hb in a laboratory is ?7% of

the target value, as suggested by the Clinical Labora- tories Improvement Act of 1988 (14). Under the con- ditions of this study, all test methods were not ex- pected to provide identical measurements so we arbitrarily chose to report the number of test samples with a >lO% difference from Coulter measured Hb as being clinically significant. There were 49 samples with a Coulter measured Hb <8 g/dL. This represents a clinically sensitive range when red blood cell trans- fusion is often considered. The incidence of over- or underestimation of the Coulter Hb summarized in Table 3 suggests that if present standards of quality control were strictly applied to the various instru- ments used under the conditions of this study, the conductivity methods would not be in conformance.

This study did not address any of the economic or practical aspects involved with bedside monitoring of Hb. In comparing the costs of disposables and equip- ment, the more portable devices such as the centri- fuge, Stat Crit@ and HemoCue@ are much less expen- sive than the Coulter counter or Nova Stat Profile 9 when fewer tests are performed. However, a device such as the Nova Stat Profile 9 is difficult to compare on a cost basis since it provides more information independent of the Hb and hematocrit that could be useful to patient management. It is also important to consider that the laboratory costs of the Coulter Hb includes quality control testing, equipment mainte- nance costs, and record keeping costs that would be necessary for the other devices as well.

We conclude that the bedside photometric and cen- trifuge methods provided the closest approximation to the Coulter Hb. Assessment of conductivity Hb with in vitro conditions exaggerated the inaccuracy of these devices, probably due to an abnormal range of sodium, chloride, and total protein not found with in vim measurements in this study. The rank order of closest approximation to the Coulter measurement was HemoCue@, centrifuge, Nova Stat Profile 9, and Stat-Crit@ during euvolemia, hemodilution, and after scavenged autologous blood transfusion.

1202 CARDIOVASCULAR ANESTHESIA MCNULTY ET AL. ANESTH ANALG BEDSIDE HEMOGLOBIN MEASUREMENTS 1995;81:1197-202

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