A systematic review 1 on the value of C-reactive protein (CRP) for the diagnosis of community acquired pneumonia (CAP) in primary care or ambulatory settings, including 8 studies with a total of 2 194 patients, was abstracted in DARE. The diagnostic utility of CRP at different cut-points was assessed against a reference standard of chest X-ray. The median prevalence of CAP in the studies was 14.6 % (range 5%-89%). At a threshold of less than 20 mg/l, positive likelihood ratio was 2.1 (95% CI 1.8 to 2.4) and negative likelihood ratio was 0.33 (95% CI 0.25 to 2.4; 6 studies). There was little evidence of heterogeneity in either outcome (p>0.15). Results at the other two CRP cut-points (≤ 50, > 100 mg/l) were heterogeneous.
A systematic review 2 including 13 studies on the diagnostic accuracy of CRP in detecting radiologically proved pneumonia was abstracted in DARE. 6 studies (1 178 patients) provided data for the evaluation. As for the diagnostic accuracy of C reactive protein in detecting radiologically proved pneumonia, the sensitivities ranged from 10 to 98% and the specificities from 44 to 99%. The area under the sROC curve for the subgroup of adults (5 studies) was 0.80 (95% confidence interval, CI: 0.75 to 0.85). Other subgroup analyses were not performed because of a lack of data. Sensitivity analysis of the areas under the curves of studies that met the four quality criteria (area under the curve 0.84, 95% CI: 0.78 to 090) and those that did not meet the criteria (area under the curve 0.74, 95% CI: 0.65 to 0.83) showed robustness of the data.
A European multi-centre diagnostic study 3 to quantify the diagnostic accuracy of selected inflammatory markers in addition to symptoms and signs for predicting pneumonia included 2 820 patients (mean age 50, 40% men) of whom 140 (5%) had x-ray verified pneumonia. The optimal combination of clinical prediction items derived from the patients included absence of runny nose and presence of breathlessness, crackles and diminished breath sounds on auscultation, tachycardia, and fever, with an ROC area of 0.70 (0.65 to 0.75). Addition of CRP at the optimal cut off of >30 mg/l increased the ROC area to 0.77 (0.73 to 0.81) and improved the diagnostic classification (net reclassification improvement 28%). With the CRP concentrations of <20, 20-30, 30-50, 50-100 the proportion of patients with pneumonia was 3%, 5%, 7%, 15%, and 35% respectively. Positive predictive values of CRP as a stand-alone test were 11.8%, 14.8%, 22.5%, and 35.4% for concentrations over 20, 30, 50, and 100 mg/L, respectively. Negative predictive values were 97.4%, 97.2%, 96.8%, and 96.1%.
Comment: The quality of evidence is downgraded by indirectness (differences between the population of interest and those studied).
Clinical comment: CRP has been investigated mainly in the hospital environment where it exhibits good sensitivity. The sensitivity is clearly lower among primary care patients (about 50%). In contrast, CRP has good specificity in primary care. Thus, CRP is a case of SpIn, i.e. a positive results is a reliable indicator of pneumonia whereas a negative result (lower than 50) is less reliable at least among primary care patients. CRP is not particularly useful in screening for pneumonia, as there will be a lot of false negative results in primary care. CRP has been less studied in patients with bronchitis, but in these patients CRP lends more support to treatment decisions since a negative CRP test does not call for antimicrobial medication.
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