It is often difficult to decide which patients with suspected lower respiratory tract infection (LRTI) should be prescribed antibiotics. Circulating calcitonin precursors are higher in severe bacterial infections than in other illnesses. In this prospective single blind study 243 patients with suspected LRTI were randomised to a standard care group (n = 119) treated with antibiotics on the basis of clinical findings and routine investigations, or a procalcitonin group (n = 124) guided by a new procalcitonin assay with a sensitivity of 0.06 μg/l. In this group, antibiotics were strongly discouraged if procalcitonin levels were <0.10 μg/l, discouraged if levels were ⩾0.10–0.25 μg/l, advised if levels were ⩾0.25–0.50 μg/l, and strongly recommended if levels were ⩾0.50 μg/l, with re-evaluation after 6–24 hours in both groups.

The primary end point was the use of antibiotics: 99 (83%) subjects in the standard care group and 55 (44%) in the procalcitonin group received antibiotic treatment. The adjusted relative risk of antibiotic exposure in the procalcitonin group was 0.49 (95% CI 0.44 to 0.55; p<0.0001) compared with the standard group. The reduction in antibiotic use was 47% in LRTI and 56% in acute exacerbations of COPD. There was no difference in laboratory and clinical outcomes including mortality, frequency and length of admission, need for ITU care, and rates of re-exacerbation of COPD and readmission after a mean (SD) of 13.0 (5.4) days. Re-evaluation after a mean (SD) of 5.3 (1.1) months showed no difference.

The fact that the study was single blind was a source of possible bias. There is no equivalent study using CRP as an alternative test. However, this study may have important clinical and economic implications.