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 o 2 = s b 2 + s a 2 . Using an equivalent equation for the Radiometer ABL, the imprecision of the two GEMs was calculated: s aGEM = (y oGEM 2 − y oABL 2 + s aABL 2 ) 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. Results Regression graphs were derived from 1800 patients' blood gas results with least 10,000 data pairs grouped into 2 h 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, pCO 2 , 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 2 pm and 2 am.