Test Background The anti FXa assay is the method of choice when monitoring low molecular weight (LMW) heparin therapy, and can be a replacement for the APTT test when monitoring therapy with unfractionated heparin. Anti-Xa may be used to monitor some patients who have ‘heparin resistance’ (ie. do not respond as expected to UFH) or who have an underlying condition or interfering factor(s) that alter the APTTR test result. Low molecular weight heparin (LMWH) therapy is usually not monitored, but it may be necessary to monitor anti-Xa in some cases, such as in patients who are pregnant, obese, very young, elderly and those who have kidney disease or dysfunction (given that LMWH is primarily cleared from the body by the kidneys). Any condition that decreases kidney function can potentially decrease LMWH clearance, increasing its concentration in the blood and increasing the potential for excessive bleeding.
Clinical Indications Monitoring heparin (especially LMWH) therapy in patients who are pregnant, obese, very young or elderly and those with renal impairment.
Sample Required 4.5 mL sodium citrate (pale blue top) adults1.8 mL sodium citrate (pale blue top) paediatrics
Sample Volume 24 hours
Notes Special handling: avoid prolonged stasis during venepuncture. Sample must be received by lab within 4 hours of collection. Please note: samples will be rejected if underfilled or overfilled.