A national survey determined critical limits used by trauma and medical centers in the United States. Mean low and high critical limits for the most frequently listed tests were the following values: glucose, 2.6 and 26.9 mmol/L; potassium, 2.8 and 6.2 mmol/L; calcium, 1.65 and 3.22 mmol/L; sodium, 120 and 158 mmol/L; hematocrit, 0.18 and 0.61; hemoglobin, 66 and 199 g/L; platelets, 37 x 10(9)/L and 910 X 10(9)/L; and white blood cell count, 2.0 X 10(9)/L and 37.0 X 10(9)/L. The high critical limit for prothrombin time was 27 seconds. Critical limits for PCO2 were 19 and 67 mm Hg; and for pH, 7.21 and 7.59. The low critical limit for PO2 was 43 mm Hg; no high critical limit was listed. The noncritical span for free calcium was 0.80 mmol/L. Important qualitative critical results included the presence of blasts on the blood smear, a Gram's stain or culture from blood or cerebrospinal fluid with positive results, and an elevated white blood cell count in the cerebrospinal fluid. A product of 15 years of collective medical judgment, these data should help physicians improve the quality and efficiency of acute patient care.