A 74 year old female has been admitted to your ICU with urosepsis. She is previously well with no previous hospital admissions. She was commenced on prophylactic subcutaneous heparin on day one of her admission and the following blood results were obtained:
|Parameter||Patient Value||Adult Normal Range|
|Haemoglobin||10.1 g/L*||9.8 g/L*||9.6 g/L*||120.0 - 160.0|
|White Cell Count||18.4 x 109/L*||14.2 x 109/L*||10.5 x 109/L||4.0 - 11.0|
|Platelet count||120 x 109/L*||101 x 109/L*||88 x 109/L*||150 - 350|
On day three, one of your trainees performs a "HITTS screen" which is reported as positive. The patient has remained clinically stable.
Describe your approach to this situation and give a rationale.
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
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
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.
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