Introduction

Thrombocytopenia is the most common haemostatic abnormality seen in the intensive care unit (ICU). The reported incidence of thrombocytopenia in this setting ranges from 15% to 60% and is more frequently seen in surgical and trauma patients (35–41%) compared to medical patients (20–25%). Approximately half of the patients with thrombocytopenia already have this on arrival at the ICU, while the remainder usually develops it in the first 4 days of being in the ICU.

Prognostic significance

The degree of thrombocytopenia has been regarded as a marker of illness severity in critically ill patients. A lower platelet count on admission to ICU correlates with higher Simplified Acute Physiology Scores (SAPS), Multiple Organ Dysfunction Scores (MODS) and Acute Physiology and Chronic Health Evaluation (APACHE) scores than those with normal platelet counts. It is also an independent predictor of mortality in ICU with the severity of thrombocytopenia being inversely related to survival. A four- to sixfold increase in mortality has been reported if the platelet count is reduced by more than 50% during ICU admission or if the thrombocytopenia is sustained for more than 4 days. Thrombocytopenia has also been associated with longer hospital and ICU stays.

Clinical presentation

Although the well-recognized clinical manifestations of thrombocytopenia are purpura, petechiae and bleeding, it is more common for these symptoms to be absent when the low platelet count is detected. Although the classical definition of thrombocytopenia is a platelet count less than 150 × 10⁹/L, significant or spontaneous bleeding rarely occurs with a platelet count above 50 × 10⁹/L (unless there are coexistent reasons for platelet dysfunction). Diffuse bleeding and haemorrhage from venepuncture sites may accompany thrombocytopenia and often due to increased vascular permeability and poor vasoconstriction rather than the low platelet count per se. At the same time, the risk of bleeding increases four- to fivefold with a count less than 50 × 10⁹/L. Spontaneous intracerebral haemorrhage is rare in ICU patients with low platelet count (frequency of 0.3–0.5%) and is most commonly seen when the platelet count is less than 10 × 10⁹/L.

In contrast to bleeding, thrombocytopenia in ICU may be the result of increased platelet aggregation in the different vasculature. Often, thrombocytopenia is an accompaniment of organ failure, especially renal impairment and respiratory distress syndrome. Platelet aggregation in the organs has been described as a contributory factor in these clinical states. In this regard, a dropping platelet count may be considered as a predictor of impending organ failure, and the aetiological causes may be sought and treated early. In addition to microvascular thrombosis, low platelet count can also be associated with an increased risk of thrombosis in disorders such as microangiopathic haemolytic anaemia or heparin-induced thrombocytopenia, where once again platelet aggregation is the underlying pathophysiological mechanism.

Another consequence of thrombocytopenia is the increased vascular permeability, which occurs with the very low platelet count. Platelets are integral constituents of the mechanisms necessary for the maintenance of the vascular integrity. Hence, in cases of thrombocytopenia, there are increased capillary leakage and consequent vascular oedema, clinically evident as generalized oedema, and adult respiratory distress syndrome.

Specific characteristics of thrombocytopenia in the ICU

• The cause of thrombocytopenia in ICU patients is usually multifactorial.
  
• Although an easily recognizable cause of drop in platelet count may be identified, several other reasons may coexist.
  
• Different reasons for thrombocytopenia may develop over the course of time – for example, drug-induced thrombocytopenia may not improve since sepsis has set in, causing thrombocytopenia due to a different mechanism.
  
• Unsuccessful management of thrombocytopenia may mean all the underlying reasons have not been adequately managed.
  
• As discussed earlier, low platelet count can present as organ failure, thrombosis and, less often, bleeding.