Description of anti-Xa monitoring practices during low molecular weight heparin use

Certain patient populations (pregnancy, cancer, renal impairment, and obesity) may be at higher risk of adverse events during low molecular weight heparin (LMWH) therapy and may benefit from anti-Xa monitoring. Yet, evidence supporting a standardized approach to anti-Xa monitoring correlated to clinical outcomes is lacking. Patients with at least one documented anti-Xa level and receiving LMWH within a 6-month period were identified. In a 6-month period, 224 adult LMWH patients with 359 anti-Xa levels were identified. Anti-Xa monitoring was most commonly performed in patients with active cancer receiving venous thromboembolism (VTE) treatment doses (57.4%) or obese patients receiving VTE prophylaxis (48.1%). Anti-Xa monitoring during renal impairment and pregnancy were infrequent (0.9% and 1.8%, respectively). Most (71.9%) anti-Xa levels were therapeutic, but only 45% were drawn correctly in relation to LMWH administration time. Compared to those with therapeutic anti-Xa levels, patients with out-of-range levels were four times as likely to receive a LMWH therapy change (odds ratio, 4.16; 95% confidence interval, 2.53-6.84). However, when levels were supratherapeutic or subtherapeutic, the LMWH doses remained unchanged in one-third to one-half of patients, respectively. Anti-Xa monitoring was most commonly performed in patients with cancer or obesity and was more common with VTE prophylaxis dosing. The majority of levels were therapeutic, indicating that anti-Xa monitoring may be unnecessary even in high-risk patient populations. Many out-of-range anti-Xa levels did not prompt a change in LMWH therapy. Further research is still needed to determine if anti-Xa- guided LMWH dosing improves clinical outcomes.