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POC-lactate: Evaluation of strip and cassette versus routine results
Abstract
Blood lactate is an indicator for metabolic acidosis and a marker for immediate intervention. Here we cross-compared two point-of-care (POC) lactate analysers with routine results: the StatStrip- Lactate and the iSTAT-1 versus the ABL-735. We also performed precision analysis. We found good correlations between the three analysers and also the precision met the validation criteria, except for pO2 and pCO2 measured with the iSTAT-1. Differences and bias were acceptable for clinical practice. The most up-to-date validation criteria were used and the pro's and con's of strip versus cassette analysers were evaluated.
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Objective. To study to what extent the fetal scalp blood lactate concentration during labor correlates with fetal scalp pH and base deficit, and metabolic acidosis at birth, and to suggest lactate cut-off values to serve as indicators for either reassurance or immediate intervention. Design. A retrospective observational study. Setting. Labor ward at a university medical center. Sample. Fetal scalp and cord blood samples with acid-base and lactate values from 486 singleton pregnancies beyond 34 weeks' gestation. Methods. The relation between lactate, pH and base deficit (BD) in fetal scalp blood was tested by Spearman's rho correlation coefficient. Lactate cut-off values indicating either reassuring fetal status or immediate intervention were estimated using percentile distribution and compared with pH and BD. Main outcome measures. Metabolic acidosis, defined as umbilical cord artery pH below 7.05 and BD calculated for the blood compartment above 12mmol/l. Results. After 127 (21%) exclusions, 486 cases were available for analysis. Fetal lactate values increased with evolving metabolic acidosis. Lactate concentration correlated with both pH (r=-0.50, p<0.01) and BD (r=0.48, p<0.01). Lactate <5.4mmol/l indicated reassuring fetal status, whereas lactate ≥6.6mmol/l indicated metabolic acidosis. Fetal lactate correlated better with either the absence or presence of metabolic acidosis at birth than did fetal pH and BD. Conclusions. In the case of a non-reassuring fetal heart rate, fetal scalp blood lactate provides more accurate information on fetal acid-base status than does pH and/or BD. © 2011 Nordic Federation of Societies of Obstetrics and Gynecology.
Background:
Although fetal blood sampling for pH is well established the use of lactate has not been widely adopted. This study validated the performance and utility of a handheld point-of-care (POC) lactate device in comparison with the lactate and pH values obtained by the ABL 800 blood gas analyzer.
Methods:
The clinical performance and influences on accuracy and decision-making criteria were assessed with freshly taken fetal blood scalp samples (n=57) and umbilical cord samples (n=310). Bland-Altman plot was used for data plotting and analyzing the agreement between the two measurement devices and correlation coefficients (R²) were determined using Passing-Bablok regression analysis.
Results:
Sample processing errors were much lower in the testing device (22.8% vs. 0.5%). Following a preclinical assessment and calibration offset alignment (0.5 mmol/L) the test POC device showed good correlation with the reference method for lactate FBS (R²=0.977, p<0.0001, 95% CI 0.9 59-0.988), arterial cord blood (R²=0.976, p<0.0001, 95% CI 0.967-0.983) and venous cord blood (R²=0.977, p<0.0001, 95% CI 0.968-0.984).
Conclusions:
A POC device which allows for a calibration adjustment to be made following preclinical testing can provide results that will correlate closely to an incumbent lactate method such as a blood gas analyzer. The use of a POC lactate device can address the impracticality and reality of pH sample collection and testing failures experienced in day to day clinical practice. For the StatStrip Lactate meter we suggest using a lactate cut-off of 5.1 mmol/L for predicting fetal acidosis (pH<7.20).
Increased blood lactate levels (hyperlactataemia) are common in critically ill patients. Although frequently used to diagnose inadequate tissue oxygenation, other processes not related to tissue oxygenation may increase lactate levels. Especially in critically ill patients, increased glycolysis may be an important cause of hyperlactataemia. Nevertheless, the presence of increased lactate levels has important implications for the morbidity and mortality of the hyperlactataemic patients. Although the term lactic acidosis is frequently used, a significant relationship between lactate and pH only exists at higher lactate levels. The term lactate associated acidosis is therefore more appropriate. Two recent studies have underscored the importance of monitoring lactate levels and adjust treatment to the change in lactate levels in early resuscitation. As lactate levels can be measured rapidly at the bedside from various sources, structured lactate measurements should be incorporated in resuscitation protocols.
Background:
Allowable analytic errors are generally based on biologic variation in normal, healthy subjects. Some analytes like blood lactate have low concentrations in healthy individuals and resultant allowable variation is large when expressed as a coefficient of variation (CV). In Ricós' compendium of biologic variation, the relative pooled intra-individual lactate variation (si) averages 27% and the desirable imprecision becomes 13.5%. We derived biologic variability (sb) from consecutive patient data and demonstrate that sb of lactate is significantly lower.
Methods:
A data repository provided lactate results measured over 18 months in the General Systems intensive care unit (ICU) at the University of Alberta Hospital in Edmonton, Canada. In total 54,000 lactate measurements were made on two point-of-care Radiometer 800 blood gas systems operated by Respiratory Therapy. The standard deviations of duplicates (SDD) were tabulated for the intra-patient lactates that were separated by 0-1, 1-2...up to 16 h. The graphs of SDD vs. time interval were approximately linear; the y-intercept provided by the linear regression represents the sum of sb and short-term analytic variation (sa):y₀=(sa²+sb²)½. The short-term sa was determined from imprecisions provided by Radiometer and confirmed with onsite controls. The derivation of sb was performed for multiple patient ranges of lactate.
Results:
The relative desirable lactate imprecision for patients with lactic acidosis is about half that of normal individuals.
Conclusions:
As such, evaluations of lactate measurements must use tighter allowable error limits.
To validate a point of care lactate device to replace fetal pH measurement.
Cord blood samples drawn immediately following delivery were tested on the Nova Lactate Plus and ARKRAY Lactate Pro, the Corometrics 220 pH System, and the Vitros chemistry analyzer (used as lactate reference).
Nova demonstrated a constant positive bias relative to the lactate reference method; while the Lactate Pro correlated well with the reference method up to 6 mmol/L. Receiver operating characteristic (ROC) curve analysis showed optimal sensitivity and specificity for predicting pH<7.20 at lactate values of 6.8 mmol/L for the Nova and 4.8 mmol/L for the Lactate Pro.
Using Lactate Pro the best cut-off for predicting pH< or =7.20 was 4.8 mM; which coincides with current clinical cut-offs. Thus any lactate device that correlates well with the laboratory reference method can be used with a clinical cut-off of 4.8 mmol/L.