F.J. HAENNI¹, M. RUSCA², E. DAYER¹, M.F. ROSSIER<sup>1

1</sup>Institut Central des Hôpitaux (ICH), Laboratoires, Hôpital du Valais, Sion, Suisse, ²Service de médecine intensive, Hôpital du Valais, Sion, Suisse

Minimal re-testing interval (MRI) is the period during which a test should not be repeated, based on the analyte properties and on the clinical situation. Establishing MRI is the responsibility of the laboratory in order to control the costs and provides assistance for requesters. Procalcitonin (PCT) is a recognized biomarker of bacterial infection useful for limiting the misuse of antibiotics. A MRI for PCT was fixed in our laboratory at 48h based on others laboratories. In a preliminary study, aimed a evaluating the cost due to early re-testing, PCT came on the 1st place, and represented 18.2% of all PCT requests. To determine what part of these early re-tests was really useful, the difference between two consecutive results was assessed. We found that 79% of PCT re-tests within 48h identified a significant biological variation. Indeed, previous studies demonstrated that PCT levels increased within 6 to 12 hours at the beginning of a bacterial infection and decreased with a half life of 24h when infection was controlled. Algorithms have been proposed in the literature to optimally use PCT for initiating an antibiotherapy and optimizing the duration of the treatment. Applying a 48h-MRI for PCT seems then inappropriate. Based on cut-offs proposed for antibiotherapy initiation, we analyzed the kinetics of PCT category transition in a series of hospitalized patients (n = 3653). It appeared that the transition of PCT from normal to pathological values, at the beginning of the infection, occurred much faster than the inverse transition. In conclusion, our data support the view that a MRI of 48h for PCT is not adapted when a decision to initiate an antibiothrerapy has to be taken. In contrast, such a MRI can be useful for limiting the cost of over-testing for monitoring the treatment efficacy after initiation. Therefore, a MRI for PCT should be adapted to the therapeutic situation. Actually, the implementation of informatics system to order laboratory tests could alert the requester if repeated measurement is within the MRI, but more effort will be done to improve the interoperability between informatics systems to provide more efficient assistance for requesters.