Background: Laboratory tests provide objective measurements of physiologic functions, but are usually evaluated by demographic reference-intervals (RI), instead of risk-based decision-limits (DL). We show that hospital electronic medical record (EMR) data can be utilized to associate all-cause mortality risks with analyte test values, thereby providing more information than RIs and defining new DLs.
Methods: Our cohort was 39,964 patients admitted for any reason and discharged alive, during two 1-year periods, at Sarasota Memorial Hospital, Florida, USA. We studied five routinely-performed in-hospital laboratory tests: serum creatinine, blood urea nitrogen, serum sodium, serum potassium, and serum chloride. By associating a mortality odds ratio with small intervals of values for each analyte, we calculated relative risk of all-cause mortality as a function of test values.
Results: We found mortality risks below the population average within these proposed DLs: potassium 3.4-4.3 mmol/L; sodium 136-142 mmol/L; chloride 100-108 mmol/L; creatinine 0.6-1.1 mg/dL; blood urea nitrogen (BUN) 5-20 mg/dL. The DLs correspond roughly to the usually-quoted RIs, with a notable narrowing for electrolytes. Potassium and sodium have reduced upper limits, avoiding a "high-normal" area where the odds ratio rises 2 to 3 times the population average.
Conclusions: Any clinical laboratory test can be transformed into a mortality odds ratio function, associating mortality risk with each value of the analyte. This provides a DL determined by mortality risk, instead of RI assumptions about distribution in a "healthy" population. The odds ratio function also provides important risk information for analyte values outside the interval.