College Answer

a) If the patient has underlying  renal failure
b)  body weight is low / incorrect dose of enoxaparin

Discussion

Anti-Xa level measurements are discussed in some detail in this article from the Australian Prescriber.

However, to answer this question, one does not need to be overfamiliar wiith the methods of anti-Xa testing. The patient has had too much enoxparin, because the level is high.

The college wants to know why he has been overdosed with clexane.

Furthermore, they have done a trough level, suggesting that they were suspecting overdose (or accumulation) to begin with. Typically, when monitoring therapeutic activity, one takes an anti-Xa level about 3-4 hours after the injection is given, to measure the peak effect.

Why might one have abnormally high Anti-Xa at trough?

Usually, this means the enoxaparin is accumulating. Either the dose was too high, or the clearance mechanisms are inadequate to remove an otherwise sensible dose.

The dose was too high:

• The weight-base dose was incorrectly calculated.
• The drug was accidentally administered twice (a common medication error - morning clexane doses are given around the time of the nursing handover)
• The weight-based dose was calculated correctly, and then the patient lost weight
• The weight-based dose was calculated correctly according to the actual body weight, but the patient is morbidly obese and clexane does not distribute into fat.
• The volume of distribution is decreased (the patient is dehydrated) - whereas the increase of Vd in pregnancy leads to subtherapeutic Anti-Xa levels in pregnancy.

The clearance mechanisms are inadequate:

• The patient has severe renal failure (creatinine clearance less than 30ml/hr)
• The patient is elderly (half-life is prolonged)