I look for the CRP level to drop to less than one-half of its value on admission after a couple of days of antibiotic therapy, since CRP half-life is less than a day following zero order elimination kinetics. Of interest, in a study of serial CRP in severe CAP (1), a CRP ratio >0.5 by day 3 was associated with non-resolving pneumonia ( sensitivity 91%, specificity 55%) performing significantly better than body temperature or WBC count. So if there is any doubt about how a patient is doing clinically, a repeat CRP looking for a drop of greater than on-half after 3-4 days of therapy may be helpful.
1. Coelho L, Povoa P, Almenda E, et al. Usefulness of C-reactive protein in monitoring the severe community-acquired pneumonia clinical course. Critical Care 2007;11:R92