C-reactive protein (CRP) levels are not a good marker of acute respiratory infections in patients with CF and will be of little use in guiding antibiotic use, a Victorian study has found.

While CRP levels have been used as a marker of bacterial infection in patients with pneumonia, they don’t have the same correlation in patients with CF, researchers at Monash University have shown.

In a study of 103 admissions for chest CT scans in patients with acute respiratory exacerbations of CF at Monash Medical Centre between 2012 and 2017, they found a wide range of CRP values and no overall pattern of high levels.

The patients in the study had an average age of 31, and significant lung disease (FEV1 % predicted of 56), with all having pathogenic bacteria in the sputum, the most common ones being Pseudomonas aeruginosa (56%) and Staph aureus (21%).

Current guidelines for pneumonia suggest elevated CRP levels of >100mg/L as a threshold for the likely need for antibiotics, but most of the patients admitted to Monash had only mildly elevated CRP levels, with an median initial value of  14  and maximum levels of 16 from an average of 3 CRP measures during the hospital stay, which averaged 18 days.

While some patients did have high CRP levels, most had levels below 20mg/L, including patients with pneumonia. Writing in the Internal Medicine Journal, the study authors noted that this is in marked contrast to conditions such as COPD, where high CRP levels have been shown to predict bacterial exacerbations.

The mild rises in CRP seen in CF patients could be because many have chronic liver disease, and also because antibiotic use prior to admission was common (94%). Chronic bacterial infection in CF patient may also blunt the immune response and biomarkers such as CRP, they added.

The clinical utility of the CRP in guiding antibiotic use in exacerbations of CF is limited,” they concluded.