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The usefulness of peripheral venous sampling in determining acid-base status in acutely ill patients was studied. A total of 171 nonarrest patients and 12 patients in cardiac arrest had paired samples of arterial and venous blood compared for correlation of blood gas results. Linear equations relating arterial and venous values of pH, PCO2, and bicarbonate were developed in both groups of patients; however, the accuracy of predicting arterial values from venous values was limited. Severe acid-base disturbances were essentially ruled out by normal or nearly normal venous blood gases. Extremely abnormal venous levels reliably reflected comparable arterial abnormalities. The results suggest that immediate intravenous bicarbonate therapy should be considered for patients with pH less than or equal to 7.05 and PCO2 less than or equal to 40 torr despite the possibility of inadvertent venous sampling. A larger series is needed to verify these results in the setting of cardiac arrest.