The value of C-reactive protein for postoperative monitoring of lower limb arthroplasty

Objectives

Determination of the utility of C-reactive protein (CRP) levels when measured 21 days after hip and knee arthroplasties for early diagnosis of infectious complications.

Method

This study was performed in two parts: establishment of a reference curve by measurement of CRP levels once a week in a cohort of 94 patients (50 total hip arthroplasties and 44 total or unicondylar knee arthroplasties); study of the diagnostic value of two different CRP cut-offs (25 mg/l, the mean CRP level and two standard deviations; 18 mg/l, mean and one standard deviation) at D21 postoperative in a population of 48 patients, of whom 12 presented septic complications (four surgical site infections [SSIs] and eight intercurrent infections).

Results

We observed very high interindividual variations in CRP values two weeks after arthroplasty. These variations decreased strongly in the third week postoperative. In the seven patients with a CRP level above 25 mg/l at D21, there were no false-positives. In the 41 patients with a CRP level below 25 mg/l at D21, there were five false-negatives and no false-positives. With the CRP threshold set at 18 mg/l, we observed four false-positives and four false-negatives.

Discussion–conclusion

A CRP level threshold of 25 mg/l is not sufficiently reliable for early detection of postoperative infections (whether at the surgical site or elsewhere), as judged by a sensitivity of 58.3% and a negative predictive value of 87.8%. However, the 25 mg/l threshold displays first-rate specificity and positive predictive values (both 100%). A CRP threshold at 18 mg/l is no better because even though it yields slightly a higher sensitivity value (66.7%), it strongly decreases specificity (88.9%). CRP is an important tool for postoperative monitoring but often appears to be difficult to use. The diagnosis of septic complications is based on clinical and paraclinical arguments. Local discharge, fever over 38 °C and local/persistent pain and stiffness are more informative indicators of postoperative infection.