In an investigation of specific haematological defects that might account for the haemorrhagic diathesis in certain smallpox patients, 93 patients with haemorrhagic and non-haemorrhagic forms of the disease were subjected to a variety of bleeding and coagulation studies. The findings indicate that smallpox patients with no clinical evidence of haemorrhage have no significant clotting defects although many have decreased platelets and clot retraction abnormality. Patients with the late haemorrhagic form of smallpox consistently show thrombocytopenia and associated abnormalities in the bleeding time, tourniquet test and clot retraction; some also have slightly depressed specific prothrombin activity.Patients with the severe, and uniformly fatal, early haemorrhagic form have severe thrombocytopenia, a marked decrease in specific prothrombin activity and prolongation of the prothrombin complex times. They also have a marked prolongation of the thrombin time, suggesting the presence of a circulating antithrombin.Both early and late haemorrhagic smallpox patients also have a marked abnormality of prothrombin consumption, indicating impaired plasma thromboplastin production. This finding could be explained by the thrombocytopenia present in all haemorrhagic cases.
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