Platelet count as a prognostic marker in intensive care

Introduction

Thrombocytopenia is a common occurrence in the critically ill population [1]. We aimed to determine the incidence, severity and prognosis of thrombocytopenia in our ICU.

Methods

A retrospective laboratory result review of 330 consecutive admissions to the Victoria Infirmary ICU was undertaken. Demographics, APACHE II score and outcome data were retrieved from the Ward Watcher system in the ICU. We compared survivors' and nonsurvivors' platelet levels on admission to the ICU and the trough level in the ICU and looked at correlation with the length of stay and duration of mechanical ventilation. Data were analysed using Student's t test, Pearson correlation coefficient and chi-squared test where appropriate.

Results

Complete data were available for 274 patients. Population demographics were as follows - 61.3% male, mean age 56.3 ± 2.1 years, median APACHE II 20 (IQR 15 to 27), crude ICU mortality 25.6% and mean length of stay 5.2 ± 0.7 days. Incidence of thrombocytopenia (platelet count <150 × 10⁹/l) was 29.8% on admission to the ICU, increasing to 46.9% when considering the entire ICU stay. Comparing survivors and nonsurvivors, nonsurvivors had a lower trough platelet count (140 × 10⁹/l vs 181 × 10⁹/l, P = 0.005). Patients with platelet counts less than 50 × 10⁹/l have the highest mortality (45.7% vs 27.6%, P = 0.006). Platelet data were used to construct an ROC curve, demonstrating an area under the curve of 0.66, P < 0.001. Platelet count correlated negatively with APACHE II (r = -0.20, P < 0.001) but did not correlate significantly with length of stay or duration of mechanical ventilation.

Conclusions

Thrombocytopenia occurs in 47% of our ICU patients. It correlates significantly with severity of illness as measured by APACHE II and decreasing platelet count correlates with increasing mortality, with the highest mortality in those with a trough ICU platelet count of less than 50 × 10⁹/l. The ROC characteristics also demonstrate that the platelet count is a useful predictor of mortality.