intensivists frequently encounter thrombocytopenia in the critically ill , which has an incidence of 30% to 50% in such patients . the low platelet count indicates the severity of disease in these patients and has prognostic significance . most often , the thrombocytopenia is a transient phenomenon , and recovery of the platelet count often reflects clinical improvement . on the other hand , a persistent low platelet count or relapse of thrombocytopenia often portends clinical deterioration and death . there are many potential aetiologies of thrombocytopenia in the critically ill , including loss , dilution , consumption , destruction and impaired production of platelets . among the various types of thrombocytopenia , heparin - induced thrombo - cytopenia ( hit ) , in which anti - heparin / platelet factor 4 complex antibodies ( the so - called hit antibodies ) play a central role , is regarded as the most common drug - induced immune - mediated thrombocytopenia . hit , which involves a paradoxical association of a procoagulant state with an anticoagulant drug , is feared because of the possible occurrence of life - threatening and potentially fatal venous and arterial thromboembolic complications . it is implicated by the association of treatment with unfractionated heparin or low - molecular - weight heparin and development of thrombo - cytopenia , both of which are common in the icu . hit is actively diagnosed because of the preventability of its sequelae . at the turn of the century seven investigations were begun at around that time , and slowly but surely the incidence of hit in the critically ill has emerged . in the preceding issue of critical care , gettings and coworkers   report their retrospective observations in surgical critically ill patients , in which they focus on the incidence of hit antibody positivity , the incidence of hit and the clinical relevance of hit antibodies . over 2 years a total of 2,046 patients were admitted , and there was suspicion of hit in 210 of these . nineteen patients tested positive for hit antibodies , yielding an incidence of 0.9% of seroconversion and hit . these patients were at increased risk for death or major thromboembolic complications and prolonged length of stay in the icu in comparison with matched control individuals . first , one can contest whether all 19 patients suffered from hit , because seroconversion does not prove that hit has developed . stimulated by the peer review process , the authors used , retrospectively , the diagnostic classification system developed by the scientific and standardization committee subcommittee on platelet immunology of the international society on thrombosis and haemostasis , which can be regarded as the current reference method . fifteen patients had an intermediate pretest probability and four patients had a high pretest probability of hit . the diagnosis of hit can be made if intermediate or high pretest probability is supported by the presence of hit antibodies and proof of platelet aggregation . in other words , laboratory confirmation of hit should consist of an antibody assay and a functional assay . in about half of cases in which there is a positive antibody test , the functional assay will be negative . gettings and coworkers   did not use a functional test , and so the reported incidence of 0.9% is the incidence of seroconversion and not necessarily the incidence of hit , which probably was lower . in the other six studies that investigated the incidence of hit antibody positivity and hit in general populations of critically ill patients [ 3 - 8 ] , similar results were reported . in 261 patients in a medical - surgical icu , no cases of hit antibody positivity and no hit were found . in 267 patients treated in a combined intensive and coronary care unit , an incidence of 0.39% ( 95% confidence interval 0.01% to 2.1% ) of hit was reported . in 55 patients treated in a general icu , an incidence of 39.5% seroconversion was detected but no cases of hit were identified . in 233 patients in a medical icu , an hit incidence of 2.5% was found . in 43 patients admitted for longer than 36 hours in a medical - surgical icu , finally , in 64 patients with multiple organ dysfunction syndrome in a surgical icu , incidences of 4.7% for seroconversion and 4.7% for hit were detected . second , the authors tried to analyze the clinical relevance of the presence of hit antibodies , but they could have chosen their control group more carefully . the control group comprised a mixture of patients , most of whom were not suspected of having hit and were therefore not tested . the authors could have drawn firmer conclusions if they had chosen the control group from among patients suspected of having hit but who tested negative for hit antibodies . nevertheless , their results are in accord with those of a large study conducted in cardiac surgery patients in the critical care setting , of whom 6.6% were positive for hit antibodies but who did not have hit . patients with hit antibodies required prolonged mechanical ventilation , developed acute renal failure necessitating haemodialysis , had a prolonged length of stay in the icu , and consumed significant additional critical care resources . the clinical relevance of the presence of hit antibodies remains unknown . other antiplatelet autoantibodies can be detected in patients with sepsis and in those undergoing cardiopulmonary bypass procedures . these auto - antibodies and hit antibodies without platelet - aggregating properties may well represent important autoimmune mechanisms in severe disease , but they may be regarded as an epiphenomenon of severe disease as well . to date , no therapy directed against antiplatelet autoantibodies is of proven value in the critical care setting . in conclusion many critically ill patients will develop a low platelet count , but the likelihood that hit is the cause is far less than that for many other causes . intensivists should employ the tools provided by the international society on thrombosis and haemostasis to diagnose hit and should combine clinical data with sufficient laboratory data . the presence of hit antibodies alone is not enough  a positive test is not necessarily a disease !
thrombocytopenia is a common finding in critically ill patients . 
 heparin - induced thrombocytopenia is an infrequent cause of a low platelet count . 
 intensivists should use the diagnostic classification system developed by the international society on thrombosis and haemostasis to diagnose heparin - induced thrombocytopenia . 
 the clinical relevance of the presence of anti - heparin / platelet factor 4 complex autoantibodies in the absence of clinical heparin - induced thrombocytopenia remains unknown .