Which patients to use POC CRP tests with?
The NICE guideline (CG191) [24] recommends using POC CRP tests to help differentiate pneumonia from self-limiting lower respiratory tract infections (LRTIs).
Current evidence supports the use of POC CRP tests (together with clinical examination) with adults with acute cough. There is also some evidence for using POC CRP tests with adults with COPD exacerbations [28]. Research evidence shows that POC CRP tests used with these patients can help safely reduce antibiotic use.
Currently there is no guideline or conclusive evidence on using and interpreting CRP tests with children and patients with infections other than cough.
It is up to individual clinicians to decide how they wish to use POC CRP tests in their consultations and in their practice as a whole.
Using POC CRP tests with patients with LRTIs
Most patients with acute cough due to LRTI consult a doctor 7 to 10 days after the initial symptoms start. Pneumonia is known to develop fast and patients can consult earlier than one week after initial symptoms. When CRP levels are lower than 20 mg/l it is a safe approach to not prescribe antibiotics in all patients regardless of illness duration. However, if a patient consults within 2-4 days of the start of the cough and their clinical presentation is severe, please bear in mind that CRP may still be rising. In this case, the cut-off values must be considered cautiously.
Nevertheless, the vast majority of patients will consult later in the illness episode, and most patients will have low CRP values.
Based on clinical presentation, if uncertainty remains:
- either plan a consultation to re-assess the patient, and CRP can be re-measured on that occasion
- or consider a delayed prescription.
Patients should be instructed to re-consult if their condition deteriorates significantly. Also, if a delayed prescription is used and there is no improvement within 48 hours, patients should re-consult.
Estimating illness severity is not easy in lower RTI. For instance, patients with pneumonia and acute bronchitis can have abnormalities on auscultation. Auscultation abnormalities alone cannot be used to reliably guide antibiotic treatment and, therefore, need to be combined and interpreted within the context of the overall clinical picture.
Considering risk factors for complicated LRTIs
While CRP tests can be used for most adults presenting with acute cough, some patients may be prone to a complicated course of LRTI. These clues, or risk factors, may be particularly helpful in patients with intermediate CRP values (20-100 mg/l) and may guide antibiotic treatment.
Risk factors for a complicated LRTI include:
- New onset mental confusion
- Respiratory rate > 30/min
- Blood pressure: systolic <90mm Hg or diastolic <60mm Hg
- Age ≥ 65
- Comorbidity:
- Use of glucocorticosteroids
- Heart failure
- Chronic obstructive pulmonary disease
- Insulin dependent diabetes
- Serious neurological conditions
- Immunosuppression
Clinical pointers which increase likelihood of pneumonia:
- Temperature ≥ 37.8°C
- Pulse > 100 beats per minute
- Crackles
- Decreased breath sounds
The more symptoms present, the greater the risk of pneumonia.
