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Procalcitonin (PCT) has proven to be a very sensitive marker of sepsis for non-leucopenic patients. Little is known about its relevance in immunosuppressed and leucopenic adults. Four hundred and seventy-five PCT determinations were carried out in 73 haematological patients: on 221 occasions the white blood cell (WBC) count was < 1·0 × 109/l and on 239 occasions it was > 1·0 × 109/l leucocytes. Patients were classified as: non-systemic infected controls (n = 280), patients with bacteraemia (n = 32), sepsis (n = 30), severe sepsis (n = 3), septic shock (n = 3) and systemic inflammatory response syndrome (SIRS) (n = 62). When the WBC count was > 1·0 × 109/l, gram-negative bacteria induced higher PCT levels (median 9·4 ng/ml) than gram-positives (median 1·4 ng/ml). In cases with a WBC < 1·0 × 109/l, PCT levels were similar for gram-negative and gram-positive bacteria (1·1 ng/ml versus 0·85 ng/ml). Regardless of the leucocyte count, the median PCT level in bacteraemia cases always remained < 0·5 ng/ml. In heavily leucopenic situations, PCT levels were never > 2 ng/ml even in the sepsis and severe sepsis/septic shock groups, whereas a WBC count > 1·0 × 109/l resulted in median PCT values of 4·1 ng/ml and 45 ng/ml respectively. The positive predictive value for sepsis (cut-off 2 ng/ml) was 93% in cases of WBC count > 1·0 × 109/l, but only 66% in leucopenic conditions. The negative predictive value (cut-off 0·5 ng/ml) was 90% when the WBC count was > 1·0 × 109/l and 63% in leucopenic conditions. Procalcitonin is an excellent sepsis marker with a high positive- and negative-predictive value in patients with WBC count > 1·0 × 109/l, but it does not work satisfactorily below this leucocyte count. [ABSTRACT FROM AUTHOR]