Anti-Xa level

There is no general population range: in someone not taking a heparin-type blood thinner the level is undetectable, and for someone who is, the laboratory prints a window that depends on which medicine was given, why, and how many hours after the injection the sample was taken.

This test measures the effect of heparin-type blood thinners (low molecular weight heparins such as enoxaparin, dalteparin and tinzaparin, and unfractionated heparin) by seeing how strongly the plasma blocks clotting factor Xa. In someone not taking one of these medicines there is nothing to measure. The number therefore reflects the medicine level rather than anything about the body itself.

Doctors request it in people taking a heparin-type blood thinner when the usual assumptions about how the medicine behaves may not hold: in pregnancy, kidney disease, very low or very high body weight, in children, or when bleeding or clotting occurs despite the medicine. It helps the doctor decide how much of the medicine is taken.

• A larger amount of the medicine, or a sample taken close to the peak (usually three to four hours after a low molecular weight heparin injection)
• Kidney disease, because the medicine is cleared more slowly
• Low body weight relative to the amount given

• A sample taken long after the injection, or just before the next one (a trough)
• A missed or late injection
• High body weight relative to the amount given
• Pregnancy, where the medicine is cleared faster

• The timing of the sample after the injection is the biggest influence, and a sample taken at the wrong time gives a misleading number
• The laboratory's calibration must match the medicine given; a result calibrated for one heparin type does not apply to another
• A zero or undetectable result is the expected finding in anyone not taking a heparin-type medicine

No fasting needed. Citrate tube. The time of the last injection must be recorded, because the result is only meaningful with it.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.