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.
Keywords: Anti-Xa level; Cancer; Enoxaparin; Low-molecular weight heparin; Obesity; Renal impairment.