Patient acuity exacerbates discrepancy between whole blood and plasma methods through error in molality to molarity conversion: "Mind the gap!".

OBJECTIVE: A mathematical constant factor is proposed to convert measured whole blood glucose molality to plasma-equivalent molarity. The objective of this study was to determine the distributions of conversion factors for groups of patients with different acuity and to assess the gap or error in plasma-equivalent glucose reporting that would occur when a mathematical constant conversion factor is used in patients.
METHODS: Distributions of hematocrit, red blood cell water and plasma water were determined in patients from the community, hospital and adult intensive care unit. Volume displacement conversion factor distributions and glucose error were determined for each group.
RESULTS: With increasing patient acuity the median hematocrit decreased, median plasma water increased and variation of these parameters increased. In hospital patients, the molality to molarity conversion factor distribution interval was 1.04-1.16, rather than a constant 1.11. Assuming direct electrode glucose devices only have error attributed to analytical imprecision (coefficient of variation of 5%), it is predicted that only 2% of community patients will have glucose results that exceed 10% of the target values. In the same device, due to variance in hematocrit and plasma water affecting the factor for conversion of molality to molarity, it is predicted that 8.2% of adult intensive care unit patients would have glucose results that exceed 10% of the target value.
CONCLUSIONS: Changes in hematocrit and plasma water concentration are predicted to affect a gap or error between whole blood direct reading biosensors and central laboratory plasma methods. This error increases and becomes more variable as patient acuity increases.