Stability of plasma anti-Xa activity in low-molecular-weight heparin monitoring

Low-molecular-weight heparin (LMWH) is now the standard of care for prophylaxis and treatment of thromboembolic disorders. Only cases with renal failure, morbid obesity or extreme age require anti-Xa monitoring to assure the therapeutic level achievement. Because of infrequent requests, the test is usually sent to the reference laboratories and specimen handling may be delayed. Because LMWHs can be kept at ambient temperature for several days, we proposed that anti-Xa levels in plasma samples are similarly steady. Patients' plasma that was requested for anti-Xa activity was left at room temperature to repeat the test 24 hours later and compare with the result of immediate assay. The study included 86 fresh specimens from 56 participants. All patients received enoxaparin with anti-Xa levels ranging from 0.1 to 2.5 U/mL. Notably, anti-Xa activities significantly rose on the second occasions (P = 8.4 x 10(-10)). The mean change of anti-Xa was +0.15 +/- 0.21 U/mL (+24.9% +/- 37.4%). Children (age <15 years) showed more marked alterations than adults (+40.9% vs. +18.2%, P = .008). There was no statistical difference in the degrees of changes between sexes and diagnoses. The data suggest that specimens sent for anti-Xa require prompt handling to prevent falsely elevated values. This observation is new and future research is needed to find the mechanism of this alteration.