Timing of HIT and rationale for platelet count monitoring following major surgery

ACCP guidelines recommend platelet count monitoring in patients with a risk of HIT that is >1% (with cancer, undergoing cardiac surgery, receiving unfractionated heparin).

• Shaded curve - Median (solid line) ±2 SD of the platelet count in 452 patients who underwent trauma surgery. After major surgery, the platelet count reaches its nadir between day 2 and day 4, followed by a reactive increase that exceeds the baseline value. Patients who are receiving medical therapy do not have this reactive increase in the platelet count, and it is sufficient to compare the platelet count at the time of clinical suspicion of HIT with the baseline platelet count (before the administration of heparin).

• Thick arrows - Days on which platelet count monitoring is appropriate (before day 1 and on day 5, 7, and 9). Because HIT typically manifests between day 5 and day 10, platelet count monitoring before day 1 and after day 10 are not required. 

• Thick solid line - Platelet counts of one individual with HIT. At the time thrombosis became evident, comparison of the actual platelet count (lower dotted line) with the pre-surgery platelet count (upper dotted line) shows only a 30% decrease (right double arrow), whereas comparison with the platelet count peak at days 6 and 7 shows the decrease of >50% (left double arrow), indicative of HIT.