A self-developing solid-phase immunoassay (B.R.A.H.M.S. PCT-Q, B.R.A.H.M.S.-Diagnostica GmbH, Hennigsdorf, Germany) has recently become available for the semi-quantitative and rapid measurement of procalcitonin (PCT). In this study we examined the validity of this assay at daily clinical routine conditions at five different hospitals in a prospective study. After development of the assay (200 microl plasma, 30 minutes incubation), PCT levels were categorized into four groups (< 0.5 microg/l; > or = 0.5-< 2 microg/l; > or = 2-< 10 microg/l; > or = 10 microg/l) according to the provided reference scale. Samples from patients with suspected elevation of PCT of different etiology (n=237) were read by various analyzers and compared with the results of the Lumitest PCT (B.R.A.H.M.S.-Diagnostica GmbH, Hennigsdorf, Germany). A total of 74.7% of measurements were categorized according to the results of the LumitestPCT, 24.5% were read within the next lower or higher category. Using a +/- 10% range at the reference concentrations (20% at 0.5 microg/l), 82.7% of samples were correctly categorized and 16.4% within the next categories. Using a cut-off value of 2.0 microg/l, 92.0% (94.1% for +/- 10%) of the results were correctly categorized. The semi-quantitative solid phase immunoassay allows a rapid, simple and semi-quantitative measurement of plasma PCT. The validity of the test results and its ease of use are sufficient to support acute diagnostic decisions. However, for the follow-up of PCT concentrations and routine daily measurements, the quantitative luminometric assay should be preferred, when available.