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

Background: Within- 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 (s_b) of blood gas (BG) and hematology analytes. We extend this methodology to derive the imprecision of two GEM 4000 BG analyzers.

Methods: A 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: y_o²=s_b²+s_a². Using an equivalent equation for the Radiometer ABL, the imprecision of the two GEMs was calculated: s_aGEM=(y_oGEM²-y_oABL²+s_aABL²)^1/2. This analysis was performed for nearly all measurements, regardless of time as well for values obtained over two 12h mutually exclusive periods, starting either at 2am or 2pm.

Results: Regression graphs were derived from 1800 patients' blood gas results with least 10,000 data pairs grouped into 2h intervals. The calculated s_aGEM exceed the directly measured s_aABL with many GEM sigma ratios of biologic variation/analytic variation being close to unity. All of the afternoon s_aGEM exceeded their morning counterparts with pH, pCO₂, K and bicarbonate being statistically significant.

Conclusion: For 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 2pm and 2am.