Sir: Heparin-induced thrombocytopenia (HIT) is one of the most serious drug reactions occurring in intensive care patients and is difficult to manage because the predictive value of the laboratory investigations is imperfect and the treatment involves anticoagulating thrombocytopenic patients at a high risk of bleeding. Since investigations for HIT have poor specificity, current American and British guidelines recommend that the probability of HIT is initially judged clinically [1, 2] and a scoring system, known as the 4 Ts score (Table 1) is recommended [3]. We performed a review of all patients at our institution who were investigated for HIT over a 6 year period. Remarkably, 70% of positive platelet aggregometry studies occurred in critically ill patients. The prevalence of HIT in our patients was similar to the reported prevalence of 0.5% [4]. We postulate that the high relative prevalence of HIT in ICU patients occurred because the incidence of HIT has fallen in other patient groups (we found it to be uncommon in postoperative orthopaedic patients although a prevalence of up to 4.9% has been reported in the literature [4]). This is probably because unfractionated heparin is now rarely used outside the context of cardiac surgery or renal failure. We analysed the predictive value of 4 Ts score in critically ill patients using 27 cases of confirmed HIT and 22 negative controls. All patients were critically ill at the time of investigation and had HIT either confirmed or excluded by a combination of platelet aggregometry and clinical assessment by a haematologist. Eleven other cases were excluded from the analysis as the clinical picture and platelet aggregometry findings were not in concordance. The predictive value of the 4 Ts score was limited in the critically ill with only 26% of patients with HIT achieving a high pretest probability (score 6–8). The score was higher in patients who had confirmed HIT (median 5 vs. 2.5, P = 0.008, Mann– Whitney U test) but the receiver operating characteristic area under the curve was only 0.69, with the optimal cut-off point of C4 giving sensitivity 69% (range 42–81%); specificity 59% (range 36–79%) and likelihood ratio 1.54. Other potential causes of thrombocytopenia were present in 85% of patients with HIT and the fall in platelet count was too early to be