Question 9.1

College answer

Probability of HITTS is low due to:

Timing of onset is too fast with no history of previous exposure

The platelet fall is not greater than 50%

There is no associated thrombosis or skin necrosis

There is a likely alternative cause – sepsis

The HITTS ELISA test has is not very specific and may give false positives

Therefore:

Reasonable to stop heparin in short term (although not mandatory)

No requirement for commencing alternatives

Could repeat test in short term

More accurate test (SRA- serotonin release test) not likely to be immediately available but will guide future management

Discussion

Let's reason through this:

• The patient has not had any heparin exposure in the last 90 days, so it's probably not the "rapid onset" HIT syndrome
• The patient has not been on heparin for the 5-10 days that it would usually take to develop the syndrome, so the timing is off
• The platelets have not fallen by the official 30% threshold (as 88 is 73% of 120)
• There's no mention of any thrombosis
• There's urosepsis, severe enough to come to ICU - which is a good explanation for the drop in platekets

So, the "4T" score is in fact 0. That's associated with the lowest possible pre-test probability for HIT.
(Here is the scoring system in case the reasoning above does not make sense)

The positive ELISA test for anti-PF4 antibodies is meaningless, because is screening studies only 2-15% of all positive patients went on to develop any sort of clinically significant HIT.

 So, the approach to this situation would be:

• Continue the prophylactic heparin 
• Ignore the test result
• Continue to monitor the platelet count
• Re-educate the trainee by referring them to the excellent NEJM article by Andreas Greinacher (2015). Somebody needs to have a word with them. Those tests aren't cheap. The serotonin functional assay and HIT ELISA together cost $194 in 2011-era Canadian money.