Laboratory markers to determine clinical significance of coagulase-negative staphylococci in blood cultures

Background

Coagulase-negative staphylococci (CoNS) are important causes of bloodstream infection, especially in immuno-compromised patients, but they are also the most common contaminants of blood cultures; discrimination between these two is often difficult. The time necessary for microbial growth to appear (time to positive) has been used as a laboratory marker for assessing clinical significance of CoNS bacteremia. However, with the use of continuously monitoring blood culture systems, the previous study results are controversial. The aim of the present study was to assess microbiological laboratory markers that are suggestive of true CoNS bacteremia.

Materials and methods

All blood cultures positive for CoNS between 1 October 2006 and 30 April 2007 in Tartu University Hospital were included in this analysis. Blood specimens were monitored with the BACTEC 9240 system. Microbes were identified using the VITEK 2 system and antibacterial susceptibility pattern was tested according to CLSI standards. The CDC definition for bloodstream infection to determine the clinical significance of CoNS was used.

Results

A total of 109 CoNS blood isolates from 86 patients (51 male; median age 22 years, range 0–94 years) were identified. According to the CDC criteria, 81 isolates were contaminants and 28 were true causes of bacteremia. Fifteen of the patients with infection were from the intensive care unit. The time to positive for blood cultures in infected patients was shorter than that in contaminated subjects: median 30 hours (range 20–68 hours) versus 42 hours (range 16–116 hours), respectively (P = 0.029). A total of 10 different CoNS species were identified; S. epidermidis was most commonly isolated in both groups – 21/28 in infection and 49/81 in contamination (OR, 1.87; 95% CI, 0.77–4.56). S. hominis comprised a higher proportion in the contamination group than in the infection group (17/81 versus 2/28 respectively; OR, 2.59; 95% CI, 0.84–8.01).

Conclusion

The majority of CoNS isolated from blood cultures were contaminants. The time necessary for microbial growth is an important laboratory marker in differentiating between true bacteremia and contamination.