From the Guidelines
For suspected pneumonia, the most essential blood test to obtain is C-reactive protein (CRP) as it strengthens both the diagnosis and exclusion of pneumonia when combined with clinical symptoms and signs. According to the Chest guideline and expert panel report 1, measuring CRP is suggested for outpatient adults with acute cough due to suspected pneumonia. The report states that a CRP of 30 mg/L, in addition to suggestive symptoms and signs, increases the likelihood that the cough may be related to having pneumonia 1.
Key Blood Tests for Suspected Pneumonia
- C-reactive protein (CRP) to aid in diagnosis and exclusion of pneumonia
- The guideline does not recommend routine microbiological testing or procalcitonin measurement for outpatient adults with acute cough due to suspected pneumonia 1.
Clinical Considerations
- The presence of symptoms such as fever (38C or greater), pleural pain, dyspnea, and tachypnea, along with signs on physical examination of the chest, can suggest pneumonia 1.
- The absence of runny nose and the presence of breathlessness, crackles, and/or diminished breath sounds on auscultation, tachycardia, and fever (38C or greater) are also suggestive of pneumonia 1.
- For patients with abnormal vital signs or suspected severe pneumonia, additional tests and imaging, such as chest radiography, may be necessary to improve diagnostic accuracy 1.
Treatment Guidance
- Empiric antibiotics may be considered as per local and national guidelines when pneumonia is suspected, especially in settings where imaging cannot be obtained 1.
- However, routine use of antibiotics is not suggested for outpatient adults with acute cough and no clinical or radiographic evidence of pneumonia 1.
From the Research
Blood Tests for Suspected Pneumonia
The following blood tests can be obtained for suspected pneumonia:
- White blood cell count (WBC) 2, 3, 4, 5, 6
- C-reactive protein (CRP) 2, 3, 4, 5, 6
- Erythrocyte sedimentation rate (ESR) 3, 5, 6
- Procalcitonin (PCT) 2
- Soluble triggering receptor expressed on myeloid cells-1 (sTREM-1) 2
- Pro-adrenomedullin (proADM) 2
- Presepsin 2
Inflammatory Biomarkers
Inflammatory biomarkers such as CRP, ESR, and WBC count can be used to differentiate between bacterial and viral pneumonia, as well as non-infectious respiratory diseases 2, 3, 4, 5, 6. However, these biomarkers are not specific for bacterial infection and can be increased by malignant diseases and collagen diseases 2.
Diagnostic Value of Blood Tests
The diagnostic value of blood tests for pneumonia is as follows:
- CRP has a sensitivity of 26% and a specificity of 83% in the screening of pneumococcal pneumonia when a cut-off limit of 60 mg x L(-1) is used 3
- Combining high CRP values (>80 mg/L) and elevated WBC count provides specificity >85% and positive likelihood ratios >3, but sensitivity <46% for both radiographic proven and bacterial pneumonia 4
- ESR and CRP have higher diagnostic odds ratios than any of the symptoms and signs 6