Pseudothrombocytopenia is an uncommon and often unrecognised phenomenon that results in errors in the interpretation of platelet counts with consequent inappropriate clinical decisions or useless, and sometimes dangerous, therapeutic interventions.
Pseudothrombocytopenia is caused by platelet clumping in vitro, which may be induced either by antibody‐mediated agglutination, the most important causes of which are ethylene‐diamine‐tetra‐acetic acid (EDTA)‐dependent agglutination and platelet satellitism, or aggregation secondary to platelet activation resulting from improper blood sampling techniques or delayed mixing with anticoagulant in the test tubes (i.e., pre‐analytical errors).
The widespread use of automatic instruments for the analysis and counting of blood cells in clinical laboratories on the one hand has enabled the recognition of this phenomenon, but on the other hand has decreased the frequency of microscopic observation of blood films, which is the only procedure that can demonstrate the presence of platelet clumps with certainty. Although almost all modern blood analysers are equipped with alarms to signal the presence of platelet clumps, these alarms are non‐specific and relatively inaccurate. In view of the practical implications of failure to recognise pseudothrombocytopenia, the best approach to the diagnosis is to have all cases of machine‐determined thrombocytopenia confirmed or excluded by microscopy.