Disagreement between ion selective electrode direct and indirect sodium measurements: estimation of the problem in a tertiary referral hospital.
OBJECTIVE
We estimated the proportion of indirect ion selective electrode (ISE) plasma sodium analyses in intensive care unit (ICU) and hospital wide, exhibiting important disagreement with direct ISE results in relation to abnormal plasma protein concentrations.


METHODS
Direct and indirect ISE plasma sodium measurements were performed on 346 clinical specimens selected to reflect low, normal, or high total protein concentrations. Important intermethod disagreement was defined as |4| mmol/L or higher. Results were extrapolated to a 3-month laboratory series of 48,033 indirect ISE assays, including 2877 samples from intensive care.


RESULTS
Intermethod sodium disagreement at |4| mmol/L or higher was predicted for 25% of ICU samples. Almost all (97%) occurred in hypoproteinemic samples where indirect tended to exceed direct ISE estimates. Hospital wide, such disagreement was projected to occur in 8% of samples, of which the majority (70%) were also hypoproteinemic.


CONCLUSIONS
Important disagreement between indirect and direct ISE sodium measurements may exist in up to 1 in 4 ICU specimens and 1 in 12 hospital-wide samples. The main problem is indirect ISE overestimation associated with hypoproteinemia, potentially leading to misclassifications of pseudohypernatremia and pseudonormonatremia. We recommend that hospital laboratories consider standardization using direct ISE sodium measurement.
Department of Chemical Pathology, Princess Alexandra Hospital, Brisbane 4102, Australia.
Goce
G
Dimeski
Thomas J
TJ
Morgan
Jeffrey J
JJ
Presneill
Balasubramanian
B
Venkatesh
D000328:Adult; D001774:Blood Chemical Analysis / Q000295:instrumentation / Q000379:methods*; D001798:Blood Proteins; D003951:Diagnostic Errors / Q000517:prevention & control*; D006801:Humans; D006955:Hypernatremia / Q000097:blood / Q000175:diagnosis*; D007019:Hypoproteinemia / Q000097:blood*; D007362:Intensive Care Units; D017736:Ion-Selective Electrodes; D015203:Reproducibility of Results; D012964:Sodium / Q000097:blood*
D001798:Blood Proteins; D012964:Sodium