Incomplete filling of lithium heparin tubes affects the activity of creatine kinase and gamma-glutamyltransferase.

This study aims to assess whether or not incomplete filling of primary lithium heparin tubes may influence the activity of creatine kinase (CK), alanine aminotransferase (ALT), aspartate aminotransferase (AST) and gamma-glutamyltransferase (GGT). Blood was drawn from 20 healthy volunteer using an identical sequence of tubes. First, a 6 mL, 13 x 100 mm 14 unit/mL lithium heparin Vacuette was filled and discharged. Then, three identical lithium heparin Vacuette tubes were filled, one to the nominal volume (i.e., full-draw tube), another with half of the nominal volume (half-draw tube) and the last with one-third of the nominal volume (low-draw tube). The plasma was separated and tested for CK (non-activated by N-acetylcysteine), AST and ALT on a Beckman Coulter Unicel DxC 800. Tests for CK were performed with a different reagent on a Beckman Coulter AU5800 (activated by N-acetylcysteine). Although the concentrations of ASL and ALT measured on the Unicel DxC and that of CK measured on the AU5800 did not change significantly across the different specimens, those of CK and GGT measured on the Unicel DxC 800 were significantly increased in the half-draw and low-draw tubes. The percentage bias of CK on the Unicel DxC 800 (using Bland Altman plots) was 3.3% and 7.9% for the half-draw and low-draw tubes, respectively, whereas that of GGT was 10.3% and 16.6% for the half-draw and low-draw tubes, respectively. These results suggest that short-draw lithium heparin tubes might be unsuitable for testing GGT and CK using specific combinations of reagents and instrumentation.