Accuracy of C-reactive protein, procalcitonin, serum amyloid A and neopterin for low-dose CT-scan confirmed pneumonia in elderly patients: A prospective cohort study

Abstract

Objective: The diagnosis of pneumonia based on semiology and chest X-rays is frequently inaccurate, particularly in elderly patients. Older (C-reactive protein (CRP); procalcitonin (PCT)) or newer (Serum amyloid A (SAA); neopterin (NP)) biomarkers may increase the accuracy of pneumonia diagnosis, but data are scarce and conflicting. We assessed the accuracy of CRP, PCT, SAA, NP and the ratios CRP/NP and SAA/NP in a prospective observational cohort of elderly patients with suspected pneumonia.

Methods: We included consecutive patients more than 65 years old, with at least one respiratory symptom and one symptom or laboratory finding suggestive of infection, and a working diagnosis of pneumonia. Low-dose CT scan and comprehensive microbiological testing were done in all patients. The index tests, CRP, PCT, SAA and NP, were obtained within 24 hours. The reference diagnosis was assessed a posteriori by a panel of experts considering all available data, including patients' outcome. We used area under the curve (AUROC) and Youden index to assess the accuracy and obtain optimal cut-off of the index tests.

Results: 200 patients (median age 84 years) were included; 133 (67%) had pneumonia. AUROCs for the diagnosis of pneumonia was 0.64 (95% CI: 0.56-0.72) for CRP; 0.59 (95% CI: 0.51-0.68) for PCT; 0.60 (95% CI: 0.52-0.69) for SAA; 0.41 (95% CI: 0.32-0.49) for NP; 0.63 (95% CI: 0.55-0.71) for CRP/NP; and 0.61 (95% CI: 0.53-0.70) for SAA/NP. No cut-off resulted in satisfactory sensitivity or specificity.

Conclusions: Accuracy of traditional (CRP, PCT) and newly proposed biomarkers (SAA, NP) and ratios of CRP/NP and SAA/NP was too low to help diagnosing pneumonia in the elderly. CRP had the highest AUROC.

Clinical trial registration: NCT02467092.

Fig 1. C-reactive protein (CRP), procalcitonin (PCT), Serum Amyloid A (SAA) and neopterin (NP) boxplot in patients with and without pneumonia.

Fig 2. ROC curves for the diagnosis of pneumonia.

a) C-reactive protein Area under the curve [95% CI] = 0.64 [0.56–0.72]. Optimal cut-off point at 109.4 mg.L^-1. b) Procalcitonin Area under the curve [95% CI] = 0.59 [0.51–0.68]. Optimal cut-off point at 1.06 μg.L^-1. c) Neopterin Area under the curve [95% CI] = 0.41 [0.32–0.49]. Optimal cut-off point at 16.4 nmol.L^-1. d) Serum amyloid A Area under the curve [95% CI] = 0.60 [0.52–0.69]. Optimal cut-off point at 15.3 μg.L^-1.