Can we reliably predict the level of anticoagulation after enoxaparin injection in elderly patients with renal failure?

OBJECTIVES: The aim of this study was to evaluate the activity of anti-activated factor X (anti-Xa) in patients with different degrees of chronic renal failure (CRF), treated with therapeutic doses of low molecular weight heparin. DESIGN: This prospective study evaluated the effect of age, renal function, BMI, gender, in determining the efficacy and safety of treatment with enoxaparin, evaluated by assessing the anti-Xa. The therapeutic anticoagulant range was set between 0.20 and 0.70 U/mL. SETTING: Two hospital geriatric units. PARTICIPANTS: 98 patients (64 men, 34 women, mean age 82 years) with CRF, treated with enoxaparin at therapeutic dosage, for deep vein thrombosis or acute coronary syndrome. MEASUREMENTS: Anti-Xa was assessed 4 h after the third administration of LMWH using Chromogenix test. Renal function was assessed by calculating creatinine clearance according to Cockcroft formula. RESULTS: The dose of enoxaparin ranged between 53 and 200 U/kg; total 4000-16000 U/day. The mean anti-Xa was 0.41 U/mL (95% CI 0.36-0.45). Multiple regression analysis selected only the dose of enoxaparin, but not age, creatinine clearance, BMI, gender, as a predictor of anti-Xa serum levels. In seven patients anti-Xa was above the range but none of them received more than 150 U/Kg enoxaparin (100 U/kg if creatinine clearance <30 mL/min). Ten patients (eight men, two women) showed suboptimal levels of anti-Xa, regardless enoxaparin dose or creatinine clearance. CONCLUSION: Enoxaparin dose reduction according to renal function decreases the risk of overdosing and potentially the risk of bleeding. The risk of under dosing seems less predictable; therefore, anti-Xa assay may be useful in severe clinical situations that require higher anticoagulant activity.