Central venous blood culture: a useful test for catheter colonisation?

Objective: To evaluate the widespread practice of using qualitative culture of venous blood taken through central venous catheters (CVCs) as a means of diagnosing catheter colonisation in situ. Design: A prospective clinical study. Patients: Three hundred fifty-one CVCs were inserted into 228 critically ill patients. Interventions: Prior to CVC removal, blood was taken for qualitative culture from the CVC and a peripheral site. All catheter tips underwent semi-quantitative analysis of bacterial colony count [1]. Measurements and results: One hundred eighteen (33.6 %) CVCs were found to be colonised on removal. A positive central blood culture had a sensitivity and specificity of 50.8 % and 78.9 % when compared with the 'gold standard' of catheter tip culture. This gives a positive predictive value of 47.7 % and a negative predictive value of 76.5 %. The sensitivity and specificity of a positive peripheral blood culture were 41.5 % and 77.7 % with positive and negative predictive values of 48.8 % and 72.9 %, respectively. When only those catheters removed because of systemic sepsis (n = 139) were considered, a positive central blood culture had a sensitivity of 58.8 % and a specificity of 69.3 %. Conclusion: Our results indicate that the use of central blood culture confers a small advantage in sensitivity compared with peripheral blood culture. This advantage was further improved by only considering the catheters removed because of systemic sepsis but at the cost of a loss of specificity. Qualitative blood culture is a poor tool for the diagnosis of in-situ CVC colonisation.