Thrombocytopenia may be associated with a variety of conditions, with associated risks that may range from life-threatening to no risk at all. At the time of initial presentation, the cause may be unclear and the direction of the platelet count trend may not be known. The clinician is faced with distinguishing among many possible causes of thrombocytopenia and determining the risks of bleeding, thrombosis, and other potential complications.
Here we discuss our approach to the adult patient with unexpected thrombocytopenia, divided according to clinical presentation (eg, asymptomatic, associated with another condition, acutely ill hospitalized patient). This approach can be used by the primary care physician and the consulting hematologist. Thrombocytopenia in neonates and children, and thrombocytopenia during pregnancy are discussed separately. (See "Neonatal thrombocytopenia" and "Causes of thrombocytopenia in children" and "Thrombocytopenia in pregnancy".)
DEFINITIONS AND AREAS OF CONCERN
What is a low platelet count — Thrombocytopenia is defined as a platelet count below the lower limit of normal (ie, <150,000/microL [150 x 109/L] for adults). Degrees of thrombocytopenia can be further subdivided into mild (platelet count 100,000 to 150,000/microL), moderate (50,000 to 99,000/microL), and severe (<50,000/microL) . Severe thrombocytopenia confers a greater risk of bleeding, but the correlation between platelet count and bleeding risk varies according to the underlying condition and may be unpredictable. (See 'When to worry about bleeding' below.)
The normal adult platelet count range is 150,000 to 450,000/microL, with mean values of 266,000 and 237,000/microL in females and males, respectively . Despite the wide variation of platelet counts within the population, variation of the platelet count in a given individual is limited. As an example, a study of serial platelet counts in 3789 individuals found that the platelet count range for an individual was relatively narrow; differences in platelet count greater than 98,000/microL occurred in less than 0.1 percent of participants . Interestingly, there was a modest seasonal variation, with platelet counts approximately 5000/microL higher in the fall/winter versus the spring/summer. This wide normal range, narrow individual range, and continuous distribution have implications for evaluating any individual patient's platelet count:
●A small proportion of the population (approximately 2.5 percent) will have a baseline platelet count lower than 150,000/microL, because normal values are typically determined based on the 95% confidence intervals around the mean.