Study objective: To assess the usefulness of serum C-reactive protein (CRP) in the diagnosis and treatment approach of patients with community-acquired pneumonia (CAP).
Design: Population-based case-control study.
Setting: A mixed residential-industrial urban area of 74,368 adult inhabitants in the Maresme region (Barcelona, Spain).
Patients: From December 1993 to November 1995, all subjects who were > 14 years of age, were living in the area, and had received a diagnosis of CAP, which had been confirmed by chest radiographs and compatible clinical outcome, were registered. Patients from residential care facilities were excluded. Serum samples were assayed for CRP in the acute phase of the disease. Data from 201 patients with CAP were compared with 84 healthy control subjects matched by age, sex, and municipality, as well as with 25 patients with initially suspected pneumonia that was not confirmed at follow-up. Median CRP levels were 110.7, 1.9, and 31.9 mg/L, respectively. The thresholds of the test for discriminating among these three groups of subjects were 11.0 and 33.15 mg/L.
Results: Eighty-nine patients (44.8%) had an identifiable etiology. The most common pathogens were Streptococcus pneumoniae, viruses, and Chlamydia pneumoniae, followed by Mycoplasma pneumoniae, Legionella pneumophila, and Coxiella burnetii. There were statistically significant differences in the median CRP levels in pneumococcal (166.0 mg/L) and L pneumophila (178.0 mg/L) etiologies compared to other causative pathogens. Lower levels of CRP were found in pneumonia caused by viruses and C burnetii as well as in negative microbiological findings. The median CRP levels in hospitalized patients were significantly higher than in outpatients (132.0 vs 76.9 mg/L, respectively; p < 0.001). Considering a cut point of 106 mg/L in men and 110 mg/L in women for deciding about the appropriateness of inpatient care, CRP levels showed a sensitivity of 80.51% and a specificity of 80.72%.
Conclusions: Serum CRP level is a useful marker for establishing the diagnosis of CAP in adult patients with lower respiratory tract infections. High CRP values are especially high in patients with pneumonias caused by S pneumoniae or L pneumophila. Moreover, high CRP values are suggestive of severity, which may be of value in deciding about the appropriateness of inpatient care.