An overlooked aspect on metabolic acidosis at birth: blood gas analyzers calculate base deficit...

AOGS MAIN RESEARCH ARTICLE

An overlooked aspect on metabolic acidosis at birth: bloodgas analyzers calculate base deficit differentlyPARISA MOKARAMI, NANA WIBERG & PER OLOFSSON

Institution of Clinical Sciences, Department of Obstetrics and Gynecology, Skane University Hospital, Lund University,Malmo, Sweden

Key wordsBase deficit, blood gases, metabolic acidosis,pH, pregnancy, umbilical cord

CorrespondencePer Olofsson, MD, Department of Obstetricsand Gynecology, Skane University Hospital,S-205 02 Malmo, Sweden.E-mail: [email protected]

Conflict of interestThe authors have stated explicitly that thereare no conflicts of interest in connection withthis article.

Please cite this article as: Mokarami P, WibergN, Olofsson P. An overlooked aspect onmetabolic acidosis at birth: blood gas analyzerscalculate base deficit differently. Acta ObstetGynecol Scand 2012; 91:574–579.

Received: 1 December 2010Accepted: 13 January 2012

DOI: 10.1111/j.1600-0412.2011.01364.x

Abstract

Objective. Metabolic acidosis (MA) at birth is commonly defined as umbilical cordarterial pH<7.0 plus base deficit (BD)≥12.0 mmol/L. Base deficit is not a measuredentity but is calculated from pH and Pco2 values, with the hemoglobin (Hb)concentration [Hb] included in the calculation algorithm as a fixed or measuredvalue. Various blood gas analyzers use different algorithms, indicating variationsin the MA diagnosis. The objective was therefore to calculate the prevalence ofMA in blood and extracellular fluid with algorithms from three blood gas analyzerbrands relative to the Clinical and Laboratory Standards Institute (CLSI) algorithm.Design. Comparative study. Setting . University hospital. Sample. Arterial cord bloodfrom 15 354 newborns. Main outcome measure. Prevalence of MA. Methods. Bloodwas analyzed in a Radiometer ABL 735 analyzer. Base deficit was calculated posthoc with algorithms from CLSI and Corning and Roche blood gas analyzers, andwith measured and fixed (9.3 mmol/L) values of [Hb]. Results. The prevalence ofBD ≥12.0 mmol/L in blood was with the CLSI algorithm 1.97%, Radiometer 5.18%,Corning 3.84% and Roche 3.29% (CLSI vs. other; McNemar test, p < 0.000001).Likewise, MA prevalences were 0.58, 0.66, 0.64 and 0.64%, respectively (p ≤ 0.02).Base deficit ≥12.0 mmol/L and MA rates were lower in extracellular fluid than inblood (p ≤ 0.002). Algorithms with measured or fixed Hb concentration made nodifferences to MA rates (p ≥ 0.1). Conclusions. The neonatal metabolic acidosis ratevaried significantly with blood gas analyzer brand and fetal fluid compartment forcalculation of BD.

Abbreviations: BD, base deficit; BDblood, base deficit in blood; BE, base excess; CLSI,Clinical and Laboratory Standards Institute; CTG, cardiotocography; ECF, extra-cellular fluid; Hb, hemoglobin; [Hb], hemoglobin concentration; MA, metabolicacidosis; STAN, ST analysis.

Introduction

During sustained hypoxia, the fetus eventually devel-ops metabolic acidosis due to anaerobic glycogenolysis.Metabolic acidosis in umbilical cord blood at birth is thusan objective measure of the fetus’s exposure and ability toresist hypoxia; therefore, metabolic acidosis is widely used asa reliable fetal outcome parameter.

Metabolic acidosis is usually defined as a low cord artery pHand a high base deficit [BD; the positive value of base excess(BE)]. The fact that BD is not a measured but a calculated en-tity from pH and Pco2 values is a problem, because differentbrands and models of blood gas analyzers use different algo-

rithms to calculate BD. This results in significantly differentBD values, although the pH and Pco2 values are the same (1).Older blood gas analyzers include a fixed hemoglobin (Hb)concentration [Hb] value in the algorithm, whereas somemodern blood gas analyzers measure the Hb concentration.This introduces another methodological problem when com-paring BD from different analyzers.

The aim of the study was to demonstrate how the preva-lence of neonatal metabolic acidosis would vary by calculatingthe rate of BD ≥ 12.0 mmol/L with algorithms from threebrands of commonly used blood gas analyzers and to com-pare this with standards recommended by the Clinical andLaboratory Standards Institute (CLSI) (2). The study also

574C© 2012 The Authors

Acta Obstetricia et Gynecologica Scandinavica C© 2012 Nordic Federation of Societies of Obstetrics and Gynecology 91 (2012) 574–579

A C TA Obstetricia et Gynecologica

P. Mokarami et al. Differences in base deficit calculation

included calculation of BD in blood (BDblood) with mea-sured and fixed Hb concentrations, and BD in extracellularfluid (BDECF).

Material and methods

Determination of umbilical cord arterial and venous cordblood gases at birth has been routine at the Skane Univer-sity Hospital in Malmo since 1981. Immediately after birthand before the newborns first breathe, the umbilical cord isdouble clamped and arterial and venous blood sampled inpreheparinized syringes and analyzed within five minutes.Since 2001, a Radiometer ABL 735 blood gas analyzer (Ra-diometer A/S, Copenhagen, Denmark) has been used. Dataare stored in the analyzer’s hard disk and can be retrievedand transferred directly to a computerized statistical pro-gram, which in this study was StatView

R©version 5.0.1 (SAS

Institute, Cary, NC, USA).Cord blood gas data were available in the analyzer hard disk

from the period 15 March 2001 to 24 September 2008. Duringthis period, there were 29 122 deliveries. The aim to determinecord blood gases routinely was obtained in 85–90% of cases,but owing to a large number of samples with the origin ofsampling not specified in the analyzer hard disk database,only 37 191 cord blood gas values marked with either “artery”(17 318) or “vein” (19 873) were identified in the database.Only singleton deliveries with pH and Pco2 values from boththe artery and vein were included in the study. To excludemix-up of vessels and cases where both samples were drawnfrom the same vessel, cases with a venous to arterial pHgradient of <0.020 were excluded. The Pco2 was not usedas a validation measure because by the electronic transfer ofdata from the analyzer to the statistical database there was norisk of mixing up pH and Pco2 values obtained from the samevessel. Arterial samples with complete data on pH, Pco2, BDand Hb concentration were included in the study.

The Radiometer analyzer works by measuring pH and Pco2

by potentiometry and Hb by spectrophotometry at a temper-ature of 37◦C (3). The analyzer was calibrated every morningby an expert. In addition to the BD values in blood obtainedfrom the Radiometer analyzer, BDblood and BDECF were cal-culated post hoc with algorithms retrieved from the manualsof the following blood gas analyzers: Radiometer ABL 735(Radiometer A/S), Corning 178 (Ciba Corning Diagnostics,Halstead, UK) and Roche cobas b 221 (Hoffmann-La Roche,Basel, Switzerland). The BD values from the three blood gasanalyzers were then compared with standards recommendedby the CLSI (2). The algorithms for calculation of BD valuesare shown in Appendix S1. Corning and Roche BDblood cal-culation algorithms (4,5; Additional supporting informationAppendix S1) include the Hb concentration at a fixed valueof 9.3 mmol/L (150 g/L), whereas the Radiometer analyzermeasures the Hb concentration and includes it in the algo-

rithm (3; Appendix S1). Thus, with the different algorithms(2–5; Appendix S1) the BD values from the Corning andRoche analyzers were also calculated with the measured Hbconcentration [Hb].

In the following descriptions of equations and algorithms,BE is reported as its negative value BD, and Pco2 values inmillimeters of mercury were transformed to SI units accord-ing to the transformation equation, mmHg = 7.5 × kPa. Inthe following text, BD for CLSI is denoted BDCLSI, for theCorning analyzer BDCorning, for the Roche analyzer BDRoche,and for the Radiometer analyzer BDRadiometer.

The Radiometer analyzer reports pH values with threedecimals, Pco2 values with two decimals, and BD with onedecimal. The pH and Pco2 values were included in full in thealgorithm when calculating BD post hoc. Base deficit valueswere then, according to the ‘round half to even’ rule, roundedoff to one decimal before calculating the rates of metabolicacidosis. Metabolic acidosis in arterial cord blood was definedas a BD ≥ 12.0 mmol/L combined with a pH < 7.00 (6).

Statistical analyzes

Simple linear regression analyzes were performed with aid ofStatView

R©computer software (SAS Institute) and categor-

ical variables were compared online with the McNemar testat the Vassar College VassarStats’ website for statistical com-putation (7). A two-tailed p-value of <0.05 was consideredstatistically significant.

The Research Ethics Committee at the Medical Faculty,University of Lund, Sweden, approved the study (approvalno. LU43998).

Results

The study included 15 354 cord arterial blood gas determi-nations with complete data on pH, Pco2, BDRadiometer andHb concentrations. Table1 shows the numbers and preva-lences of BD ≥ 12.0 mmol/L as calculated with CLSI andblood gas analyzer algorithms. For BDblood, the prevalencewas significantly higher when calculated with the three differ-ent analyzer algorithms compared with the CLSI algorithm.At inter-analyzer comparisons, all BDblood ≥ 12.0 mmol/Lprevalences (for both measured and fixed Hb calculations)and BDECF ≥ 12.0 mmol/L prevalences were significantly dif-ferent. Intra-analyzer and “intra-CLSI” comparisons showedthe same results. Maximum differences at inter-analyzer com-parisons were 163% (a highest-to-lowest prevalence ratio of2.63) for BDblood (Radiometer vs. CLSI) and 210% (ratio 3.1)for BDECF (Roche vs. Corning); for intra-analyzer compar-isons the maximum figure was 426% (ratio 5.26, measuredHb BDblood

Corning vs. BDECFCorning).

Owing to the inclusion of measured Hb in the BDbloodCLSI

and BDbloodRadiometer algorithms, regression analysis showed

a scattering of values (Figure 1). Calculated by the regression

C© 2012 The AuthorsActa Obstetricia et Gynecologica Scandinavica C© 2012 Nordic Federation of Societies of Obstetrics and Gynecology 91 (2012) 574–579 575

Differences in base deficit calculation P. Mokarami et al.

Table 1. Prevalence of base deficit (BD) ≥ 12.0 mmol/L in umbilical cord blood at birth calculated with four different BD algorithms in a population

sample of 15 354 deliveries.

Algorithm/analyzer BDblood with measured [Hb]BDblood with

[Hb] = 9.3 mmol/L BDECF

n Percentage n Percentage n Percentage

CLSI 303a 1.97 – – 267c 1.74Radiometer 796a 5.18∗ – – 167c 1.09∗

Corning 589d 3.84∗ 505b 3.29∗ 112c 0.73∗

Roche 505d 3.29∗ 450b 2.93∗ 347c 2.26∗

Base deficit was calculated in blood (BDblood) and extracellular fluid (BDECF), using both the meaured hemoglobin (Hb) concentration and a fixed [Hb]of 9.3 mmol/L (150 g/L) according to the algorithms described in the analyzer manuals. For description of blood gas analyzers and algorithms, seemain text. Abbreviation: CLSI, Clinical Laboratory Standards Institute.aOriginal analyzer algorithm includes meaured [Hb] for calculation of BDblood.bOriginal analyzer algorithm includes a fixed [Hb] of 9.3 mmol/L for calculation of BDblood.cFor the Radiometer analyzer, a fixed [Hb] of 3.0 mmol/L was used in the algorithm; for other analyzers [Hb] is not included in the BDECF algorithms.dPost hoc calculations using measured Hb concentration.∗Comparisons with CLSI, McNemar test, p < 0.000001.

equations, a BDbloodCLSI value of 12.0 mmol/L corresponded

to a BDbloodRadiometer value of 14.1 mmol/L (Figure 1) and a

BDbloodRoche value of 12.7 mmol/L (figure not shown).

The prevalence of pH < 7.00 was 0.68%, correspondingto 104 cases. In Table2, the prevalences of metabolic aci-dosis (pH < 7.00 and BD ≥ 12.0mmol/L) with regard toBD algorithm are displayed. The prevalences were signif-icantly higher with the analyzer BDblood algorithms thanwith the BDblood

CLSI algorithm. At interanalyzer compar-isons, metabolic acidosis prevalences with BDECF calculationswere all significantly different, but with BDblood calculationsonly the differences to BDblood

Radiometer calculations were sig-nificant. At intra-analyzer comparisons, the differences be-tween BDblood and BDECF calculations were significant, butcalculation of metabolic acidosis in blood with measured orfixed Hb made no statistical difference.

Discussion

Neonatal cerebral dysfunction frequently occurs with an um-bilical artery BD greater than 12.0 mmol/L at birth (8) andthis threshold is commonly used together with a low pH asa cut-off to define metabolic acidosis. In the literature, BD ispresented as a well-defined entity, but our study demonstratesthat the rate of values above this threshold varies significantlywith different blood gas analyzers. In comparisons with theCLSI standard, the rate of BDblood values ≥ 12.0 mmol/L wasup to 163% higher when calculated with the analyzer algo-rithms, and for BDECF it varied between 42% lower and 30%higher. To demonstrate the impact of different algorithmsfurther, a BDblood

CLSI value of 12.0 mmol/L corresponded toa BDblood

Radiometer value of 14.1 mmol/L and a BDbloodRoche

value of 12.7 mmol/L.

This demonstrates a problem that has previously attractedlittle attention in the obstetric literature. The calculation ofBD varies between the blood gas analyzers owing to the useof different solubility factors for CO2, the slope of the CO2

equilibration line for whole blood, the plasma bicarbonateconcentration at reference pH, and the different standardsto include the actual oxygen saturation in the calculationalgorithm. Kofstad (9) calculated BDblood and BDECF with al-gorithms from different blood gas analyzers in 81 simulatedclinical situations with acid–base disturbances in adults, andfound that within the range with normal to acidotic pH val-ues the maximum BDblood difference was 1.2 mmol/L. ForBDECF, the difference was 0.2 mmol/L. Kofstad stated thatsuch differences are clinically unimportant. However, in ourseries of 15 354 umbilical cord arterial blood samples thedifference was often 3–4 mmol/L at normal and moderatelyhigh BDblood values, and at extremely high values it was oc-casionally 8–9 mmol/L (Figure 1). Such large variations dueto methodological differences between blood gas analyzersmight result in active neonatal treatment in one situationand conservative management in the other.

Base deficit values in umbilical cord blood vary not onlywith the calculation algorithm, but are also influenced bygestational age, length of labor, fetal fluid compartment forcalculation and time of blood sampling after birth (1,10,11).In a previous study, we calculated, post hoc in the same bloodsample, the BD in blood to be 1–2 mmol/L higher than inextracellular fluid (1). This explains why, in the present study,the intra-analyzer rate of BD ≥ 12.0 mmol/L differed by upto 426%, i.e. a highest-to-lowest prevalence ratio of 5.26,when calculating the prevalence in blood and extracellularfluid.

The BD ≥ 12.0 mmol/L prevalence differences had a sig-nificant impact on diagnosis of metabolic acidosis, for both




Table 2. Prevalence of metabolic acidosis (pH < 7.0 and BD ≥ 12.0 mmol/L) in arterial umbilical cord blood at birth calculated with four different BD

algorithms in a population sample of 15 354 deliveries.

Algorithm/analyzerMetabolic acidosis in blood,

measured [Hb]Metabolic acidosis in blood,

[Hb] = 9.3 mmol/LMetabolic acidosis in

extracellular fluid

n Percentage n Percentage n Percentage

CLSI 89 0.58 – – 70 0.46Radiometer 102 0.66∗ – – 62 0.40∗

Corning 98 0.64∗ 97 0.63∗ 46 0.30∗

Roche 98 0.64∗ 94 0.61† 77 0.50∗

∗ Comparisons with CLSI, McNemar test, p < 0.02.† Comparison with CLSI, McNemar test, p = 0.06.

Figure 1. Relation between base deficit in blood (BDblood) calculatedwith the Clinical Laboratory Standards Institute (CLSI) algorithm and theRadiometer algorithm, which both include the measured hemoglobinconcentration. The correlation coefficient was 0.990 at simple linear re-gression analysis (y = 0.161 + 1.162x). The line of identity is indicated. ABDblood

CLSI value of 12.0 mmol/L corresponded to a BDbloodRadiometer value

of 14.1 mmol/L, and a BDbloodRadiometer of 12.0 mmol/L to a BDblood

CLSI of10.2 mmol/L.

BDblood and BDECF. With the CLSI standard, the prevalence ofmetabolic acidosis, defined as BDblood ≥ 12.0 mmol/L com-bined with a pH < 7.00 in cord arterial blood, was 0.58%,which was significantly lower than the figures of 0.64–0.66%calculated with the blood gas analyzer algorithms. In abso-lute figures, the difference was at the most 13 cases among104 newborns with a cord arterial pH < 7.00. In analyzerswith a fixed [Hb] value in the algorithm, e.g. the Corning andRoche analyzers, replacing the fixed [Hb] value of 9.3 mmol/L

(150 g/L) with the measured Hb concentration resulted insignificantly different BDblood values, but when compared formetabolic acidosis prevalence it made no statistical differ-ence.

The metabolic acidosis rate decreased markedly whenBDECF was calculated instead of BDblood; from 0.58 to 0.46%with the CLSI standard, and from 0.64–0.66 to 0.30–0.50%with the analyzers. A high Pco2 indicates a considerable in-fluence of the respiratory component in the BD algorithm,and Rosen & Murphy (12) advocate the use of BDECF inperinatal medicine because hypercapnia is common in new-borns and BDECF is influenced by Pco2 to a lesser degree thanBDblood. Moreover, the fetus has a proportionally large extra-cellular compartment, representing a considerable bufferingcapacity. In contrast, Morgan et al. (13) found BDblood to belittle affected by Pco2 or low Hb. A puzzling factor is thatthe algorithms used by blood gas analyzers are based on theadult fluid compartment distribution. Roemer (14) thereforesuggested that if estimates of the metabolic component areused, which is not self-evident because pH associates withCTG pattern and Apgar score better than BD (1,14), BDECF

should be used with correction to 100% oxygen saturation offetal Hb; however, Roemer stated that the correction seemsclinically irrelevant (14).

Determination of cord blood gases and metabolic acidosisis important for the following reasons: to evaluate the courseof labor and management in individual cases; to judge mal-practice in litigation cases; to assess the quality of care ata maternity unit; and in research as an objective perinataloutcome parameter. A revealing example of how a scientificresult can be altered by using different algorithms to calculateBDECF has been reported in the literature (15). In a Finnishstudy on the value of fetal ECG for fetal surveillance in la-bor, i.e. ST segment analysis (STAN), the rate of metabolicacidosis was 0.7% in the group monitored with cardiotocog-raphy (CTG) alone and 1.7% in the group monitored withCTG + STAN when BDECF was calculated with an algorithmidentical to the Roche algorithm in the present study (16);


Differences in base deficit calculation P. Mokarami et al.

when recalculated with the BDECF algorithm used in mostother STAN studies (15,17–22; in fact, it is the BDECF

Corning

algorithm), the rates decreased to 0.6 and 0.8%, respectively(22). In a recent Dutch randomized controlled study on CTGalone vs. CTG + STAN for intrapartum monitoring (23), asignificant reduction in metabolic acidosis at birth was foundin the CTG + STAN group when metabolic acidosis was cal-culated according to a BDblood algorithm, but not with thecorresponding BDECF algorithm (in fact the BDCorning algo-rithms).

Thus, analyzers with different algorithms to calculate BDwill report different values of BD in an analyte. There is nosolid scientific basis or international consensus on which BDalgorithm is the best to reflect intrauterine acid–base statusor predict short-term and long-term outcomes. Put simply,the very large differences in rates of abnormal BD values pro-duced by different blood gas analyzers have not previouslybeen recognized. The methodological confounding raises aquestion about BD as a sound marker for fetal exposure andresponse to hypoxia. In a recent study, we found that lacticacid (lactate) had a better accuracy than BDblood to reflect afive minute Apgar score < 7, both with regard to crude val-ues and to gestational age-adjusted values (24). Moreover, incombinations of parameters the gestational age-adjusted pHplus lactate values were slightly better than the age-adjustedpH plus BDblood values. Lactate is the main end-product inanaerobic metabolism and might be an alternative to esti-mate the fetal ‘oxygen debt’. In contrast to BD, lactate is ameasured value and not an estimate calculated from otheracid–base parameters. Lactate production is the major con-tributor to a BD increase during hypoxia, and a recent animalstudy indicates that the lactate production during aggravat-ing hypoxia represents an accurate assessment of the fetalmetabolic acidosis status (25).

In summary, this study pinpointed the methodologicalconfounding in calculating BD by different blood gas an-alyzers and reported the consequences for the diagnosisof metabolic acidosis at birth. The interanalyzer rate ofBD ≥ 12.0 mmol/L differed by up to 163% for BDblood

and 210% for BDECF. The intra-analyzer rate difference forBDblood vs. BDECF ranged up to 426%. In addition, there werestatistically significant differences in the rates of diagnosis ofmetabolic acidosis in all but one comparison between theCLSI and the analyzers. The clinical and scientific implica-tions of these findings are obvious; neonatal metabolic aci-dosis rates cannot be compared between maternity units andbetween scientific articles where different fetal compartments(blood or extracellular fluid) and different algorithms to cal-culate BD have been used. In a computerized milieu, it wouldbe easy to calculate a defined BD value post hoc, as in thepresent study, but further studies are needed to clarify whichare the best cord blood parameters to indicate hypoxia andmetabolic acidosis in the human fetus and newborn.

Funding

This study was supported by grants from Region Skane andthe Medical Faculty at the Lund University (ALF).

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Supporting Information

Additional Supporting Information may be found in the on-line version of this article.

Appendix S1. Algorithms for calculation of base deficit(BD) according to the Clinical and Laboratory StandardsInstitute (CLSI), and to the Radiometer ABL 735, Corning178 and Roche cobas b 221 blood gas analyzers.


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