Heparin Concentration in Evacuated Tubes and Its Effect on pH, Ionized Calcium, Lactate, and Potassium in Venous Blood Gas Analysis.

Arterial blood specimens collected in evacuated tubes are unacceptable for blood gas analysis. However, evacuated tubes are routinely used for venous blood-gas analysis. The impact of the blood to heparin ratio on venous blood in evacuated tubes is unclear. Venous blood was drawn into lithium and sodium heparin evacuated tubes that were 1/3 full, ½ full, 2/3 full, and fully filled. Specimens were analyzed for pH, ionized calcium (iCa), lactate, and potassium on a blood-gas analyzer. The results for specimens filled only 1/3 full for lithium and sodium heparin tubes revealed a significant increase in pH and a significant decrease in the iCa. Underfilling the lithium and sodium heparin evacuated tubes did not significantly impact the lactate or potassium results. Venous whole-blood specimens should be filled to at least 2/3 full for accurate pH and iCa results.