Hello all. Thank you for your comments on investigations for HIT. As mentioned by a number of contributers there are a number of tests available when investigating a patient for possible HIT. These are summarised in a comment from a Newcastle BMS (you may want to check with your local lab as to the tests they use).
Back to the case…The patient is tested, initially using a rapid gel agglutination assay. This is positive. However this is a test with a good negative predictive value but poor positive predictive value due to a lack of specificity. Therefore further testing was performed by ELISA and was also positive.
QUESTION FOR THE TEAM:
What anticoagulant would do you use? How do you monitor this and what problems do you encounter?
Comments on HIT screening
An initial HIT screen using a rapid gel particle agglutination method (Biorad/Diamed) in which polymer particles coated with heparin/PF4 are mixed with patient serum in a small gel ID card. After 5 minute incubation the card is centrifuged for 10 min and the results interpreted visually. In a HIT positive result the particles agglutinate and remain on the top of the gel, negative test results in particles being centrifuged to the bottom of the gel.
We also have a second line test Enzyme Immuno assay ELISA IgG (GTI) which we perform on any HIT screen tests which are positive which takes about 2 hours to perform. The ELISA test uses micro plate strips coated with PF4-PVS which will bind with any HIT antibody present, a second antibody is added coated with a marker which binds to the HIT PF4-PVS complex, substrate is added and the marker produces a colour change proportional to the amount of HIT IgG antibody bound to the micro plate well, the OD(optical density) is determined using a micro plate spectrophotometer reader. The second confirmation part of the GTI ELISA assay utilises heparin to determine if the positive result is inhibited by heparin – to make the test more specific for HIT antibodies.