D-Phenylalanyl-L-prolyl-L-arginine chloromethyl ketone (PPACK), a selective thrombin inhibitor, was evaluated as an alternative anticoagulant to lithium heparin (LiHep) for blood gas and whole-blood electrolyte analyses. Initial studies in serum revealed a negative proportional bias for ionized calcium (iCa) concentration measured in the range of increasing concentrations of LiHep (-0.02 mmol/L iCa per 33 kIU/L), whereas no bias in iCa concentration was detected with increasing concentrations of PPACK. No anticoagulant-dependent changes in serum sodium, potassium, chloride, glucose, lactate, ionized magnesium, or pH were detected with either PPACK or LiHep. Similarly, no bias was observed in the measurement of whole-blood PO2, PCO2, pH, or sodium or potassium concentrations with either anticoagulant. Whole blood anticoagulated with LiHep (33 kIU/L) demonstrated a consistent -0.06 +/- 0.01 mmol/L bias for iCa compared with a nonanticoagulated whole-blood control. In comparison, no iCa bias was observed with PPACK-anticoagulated whole blood. We conclude that PPACK has ideal bias-free properties as an anticoagulant in analyses for blood gases, electrolytes, and iCa.