Question 1 · Hard
An 83-year-old woman weighing 47 kg is treated for a pulmonary embolism with enoxaparin 1 mg/kg twice daily. Her creatinine clearance is 21 mL/min. After the fourth dose she develops a large thigh hematoma, and her hemoglobin falls from 118 to 92 g/L (11.8 to 9.2 g/dL). An anti-Xa level, calibrated for LMWH and drawn 4 hours after that dose, is 1.72 IU/mL. The laboratory's peak targets are 0.5-1.0 IU/mL for twice-daily dosing and 1.0-2.0 IU/mL for once-daily dosing. How should the anti-Xa result be interpreted?
- ASupratherapeutic, from renal accumulation
- BUninterpretable, as timing is wrong
- CTherapeutic, by the once-daily target
- DTherapeutic, as PE needs higher levels
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Answer: A · Supratherapeutic, from renal accumulation
Four hours after a dose is the correct time for a peak level, and after the fourth dose the drug is at steady state, so the result can be interpreted. On twice-daily dosing the target is 0.5-1.0 IU/mL, and 1.72 IU/mL is well above it; the once-daily range does not apply to her regimen. Enoxaparin is cleared mainly by the kidneys, so it accumulates when creatinine clearance is below 30 mL/min, which is why the usual treatment dose is reduced to once daily in this group. The bleed fits the high level, and the team should be told at once so the next dose can be withheld and reduced. Anti-Xa monitoring is most useful in exactly these patients: renal impairment, extremes of body weight, pregnancy and children.
- B: Four hours after the fourth dose is a correctly timed steady-state peak.
- C: She is on twice-daily dosing, so the 0.5-1.0 IU/mL target applies.
- D: The same target applies to PE; 1.72 IU/mL is well above it.