[What does neutropenia tell about the existence of sepsis in the newborn? Predictive value of clinical laboratory tests].

A laboratory test can have one of three different functions. As a confirmatory test it can prove a diagnosis, as a screening test it can indicate the probability, that a disease in question is actually present, and as a monitoring test it can be used to guide a medical therapy. To assess the clinical value of a screening test the criteria of sensitivity, specificity and predictive value are commonly used. The meaning of these terms is explained using the example of neutropenia as a screening test for sepsis on the first day of life. For the neonates admitted during their first day of life to the neonatology units of the obstetric and paediatric departments of Zurich University a neutrophil count below 3 X 10(9)/1 as a test for sepsis has a sensitivity of 67%, a specificity of 96.7%, a predictive value of the positive test of 20%, and a predictive value of the negative test of 99.6%. The predictive value of a test is strongly dependent on the prevalence of the disease looked for in the population under study. Variation of the cut off point of a screening test results in a change of its sensitivity and predictive value of the positive test in the opposite direction.