Spectrophotometric heparin assays are expected to be independent on clotting factors, either activated or nonactivated, but could be sensitive to heparin neutralization during blood collection. Is was shown that more than 200 U of heparin/1 plasma can completely be neutralized during blood processing. Because the heparin neutralization is not constant but dependent on the sample as well as on the type of sample handling, one cannot beforehand compensate and correct the results for possible heparin neutralization. We tested the use of pyridoxal 5’-phosphate (PLP) to prevent heparin neutralization. As the use of PLP in the citrate tube decreased the heparin neutralization to a negligible effect, PLP-citrate tubes are to be preferred for all plasma heparin determinations.