The effect of the order of filling tubes after venipuncture on serum potassium, total protein, and aspartate and alanine aminotransferases.

The serum components mentioned were measured from the first and last tubes filled with blood following one venipuncture performed without use of a tourniquet on ordinary outpatients. The concentration of potassium was significantly higher in the first tube, whereas the concentration of the other components did not change. In a second series of experiments the first drops of blood (approximately 1 ml) emerging from the needle were collected separately. In the serum from this tube, the potassium concentration was significantly higher than in the serum from the second tube (approximately 10 ml), whereas the sodium concentration was the same. It is concluded that serum potassium should routinely be assayed from the second or a later tube of blood filled after one venipuncture. The S-ASAT, S-ALAT and S-total protein may be assayed from any tube.