Aim: To evaluate quantitative blood culture as a secondary test on a positive blood culture for the diagnosis of sepsis in newborn infants.
Method: A 15-month prospective study of colony forming units (CFU) on positive blood cultures from newborn infants clinically suspected of having bacterial sepsis. Growth of bacteria in peripheral blood cultures was quantified using the isolater 1.5 microbial tube lysis direct plating culture system. Colony forming units were evaluated against a clinical assessment of infection.
Results: Of 137 positive blood cultures, 71 (51.8%) were taken from neonates with clinically defined infection and 66 (48.2%) were from non-infected infants. The clinical and biographical data in these two groups were similar. Coagulase negative staphylococci were the most commonly isolated organisms in each group (60.6% vs 93.9%). Eight deaths from sepsis occurred in the clinically infected group. Eighty-five per cent of sepsis was late onset. Although a CFU count > or = 30/mL predicted sepsis (sensitivity 83%, specificity 60%, positive predictive value 69%, negative predictive value 76%), a CFU count < 30/mL did not rule out serious sepsis. The higher the CFU count the greater the likelihood of sepsis.
Conclusion: Quantitative blood culture was not shown to be a sensitive secondary test on a positive blood culture to distinguish clinical sepsis from culture contamination. Although a positive threshold of > or = 30 CFU/mL proved to be optimal, improvement in test performance would be expected with a lower incidence of culture contamination.