Impaired clinical utility of sequential patient GEM blood gas measurements associated with calibration schedule

- Typical evaluations of tandem critical care instruments may not adequately detect between analyzer variation.
- Serial patient data originating from a group of analyzers can be used to describe the usual imprecision of laboratory data encountered by the clinician.
- If two intensive care units are similar in terms of patient morbidity, but use dissimilar instrument sets, the analytic error rendered by one set of analyzers may be derived algebraically from the analytic imprecision of the other set of analyzers.
- The sigma values of multiple analytes measured by tandem GEM 4000 systems are close to 1 indicating unacceptably high between instrument variation.
- This high between instrument variation increases with the time elapsed after running PCS C, a calibrator which is usually run into the early morning.

BackgroundWithin- and/or between-instrument variation may falsely indicate patient trends or obscure real trends. We employ a methodology that transforms sequential intra-patient results into estimates of biologic and analytic variation. We previously derived realistic biologic variation (sb) of blood gas (BG) and hematology analytes. We extend this methodology to derive the imprecision of two GEM 4000 BG analyzers.MethodsA laboratory data repository provided arterial BG, electrolyte and metabolite results generated by two GEM 4000s on ICU patients in 2012-2013. We tabulated consecutive pairs of intra-patient results separated by increasing time interval between consecutive tests. The average between pair variations were regressed against time with the y-intercept representing the sum of the biologic variation and short term analytic variation: yo2 = sb2 + sa2. Using an equivalent equation for the Radiometer ABL, the imprecision of the two GEMs was calculated: saGEM = (yoGEM2 − yoABL2 + saABL2)1/2. This analysis was performed for nearly all measurements, regardless of time as well for values obtained over two 12 h mutually exclusive periods, starting either at 2 am or 2 pm.ResultsRegression graphs were derived from 1800 patients' blood gas results with least 10,000 data pairs grouped into 2 h intervals. The calculated saGEM exceed the directly measured saABL with many GEM sigma ratios of biologic variation/analytic variation being close to unity. All of the afternoon saGEM exceeded their morning counterparts with pH, pCO2, K and bicarbonate being statistically significant.ConclusionFor many analytes, the average analytical variation of tandem GEMs approximates the biologic variation, indicating impaired clinical usefulness of tandem sequential measurements. A significant component of this variation is due to increased variation of the GEMs between 2 pm and 2 am.