In this study, we were able to confirm the usefulness of meningococcal PCR in clinical practice in a tertiary-care pediatric hospital. Retrospective laboratory studies of meningococcal PCR have demonstrated high sensitivities (89 to 91%) and specificities (91 to 100%) for both CSF and blood (5
). However, there have been very few prospective studies in a clinical setting. In earlier studies, the sensitivity of PCR was found to be higher than that of blood culturing (47 versus 31%), and the sensitivity increased to 88%, with improved specificity, when PCR was used with whole blood rather than serum in a subsequent group of patients (7
Since it is more sensitive than culturing, PCR has also been used to investigate clinically suspicious culture-negative cases both in the laboratory and in vaccine trials, increasing the number of confirmed cases of meningococcal disease by up to 60% (24
). In our study, an additional eight (culture-negative) cases were confirmed by PCR—a 33% increase. The increased yield of PCR is likely to be greater when higher proportions of sick children receive antibiotics before hospital admission.
PCR has the further potential advantage of providing more rapid confirmation of the diagnosis than culturing. In this study, PCR results were often available on the day of presentation, compared to the 1 or 2 days (or more) required for culture confirmation. In a number of cases, this situation allowed clinicians to target antibiotic treatment (e.g., to penicillin from broad-spectrum cephalosporin antibiotics) and to limit investigations for other diagnoses.
The duration after antibiotic administration during which meningococcal PCR remains positive was not previously determined prospectively. One retrospective survey showed that no blood PCR results were positive when the samples were taken more than 24 h after antibiotic administration, although CSF PCR results may remain positive for up to 72 h (28
). In this study, we found that blood PCR results may remain positive for up to 9 days after the administration of systemic antibiotics, and in a third of patients who had repeat testing, these results were still positive at 72 h. This finding may reflect bacterial load, since DNA persists after bacteria have been killed and higher levels may take longer to decline. Hackett et al. showed that bacterial load at presentation correlates with disease severity (18
). Although they did not show a correlation among bacterial load, duration of clinical symptoms, and decline in DNA load, the patient in our study whose blood PCR results were still positive 9 days after the start of antibiotic treatment had the longest duration of clinical symptoms and the most severe complications. This finding suggests that it may still be worth performing meningococcal PCR even 3 or 4 days after the start of antibiotic treatment in patients with delayed or unusual clinical presentations or when the initial sample is lost or insufficient.
This is the first prospective study to compare two PCR methods with the same patients. The results were concordant for all of the positive nested PCR results. Real-time PCR was more sensitive than nested PCR and confirmed two further cases of meningococcal disease. Meningococcal disease was confirmed by culturing in 63% of patients with a gold standard diagnosis of disease, in 88% of patients by nested PCR, and in 96% of patients by real-time PCR. Furthermore, real-time PCR results were positive for CSF supernatants and for longer after antibiotic administration, suggesting that real-time PCR may be more sensitive than nested PCR when the bacterial DNA load is lower. Ultimately, however, the real advantage of real-time PCR over nested PCR is the rapidity with which results can be made available.
There are no clear guidelines for the use of meningococcal PCR. We aimed to identify a subgroup of high-risk children for whom the test may be targeted, in order to avoid the impracticality and expense of performing PCR in every febrile child admitted to a hospital but without missing cases, especially those with occult meningococcal bacteremia, for whom complication rates are high (21
). We identified features at admission that classified patients as having a moderate suspicion of meningococcal septicemia or bacterial meningitis. Of the 54 patients in this group, 15 had positive cultures for N. meningitidis
and 23 had positive PCR results (blood, CSF, or joint effusion). Of the 64 patients without these features, none had positive cultures for N. meningitidis
and only 1 had a positive PCR results. The latter patient was thought unlikely to have meningococcal disease at admission; these data demonstrate that not all children with meningococcal bacteremia are significantly ill. No other patient in this group proved to have meningococcal disease. We did not investigate the rate of positive meningococcal PCR in healthy children, although up to 17% of healthy children have detectable pneumococcal DNA in their blood (14
). However, with no false positives in 118 febrile children, we consider this not to be a major problem. The yield of PCR performed on samples from every child who could possibly have meningococcal disease (i.e., any child admitted with a fever) is low. We therefore propose that patients should be targeted for meningococcal PCR on the basis of the following clinical features at admission: fever and petechiae, sepsis with shock or ICU admission, and signs of meningitis. In addition, real-time PCR may be particularly useful when the bacterial load is low.
We continue to recommend conventional microscopy and culturing in addition to PCR testing. Culturing is still the gold standard for serological classification both for investigation of outbreaks by DNA fingerprinting and for consideration of vaccine prophylaxis. In addition, culturing allows for complete antimicrobial susceptibility testing of isolates, whereas the utility of PCR for predicting susceptibility profiles has yet to be widely validated (3
). Although in Australia there has yet to be an isolate of N. meningitidis
resistant to penicillin, increasing penicillin MICs and the emergence of beta-lactamase-producing organisms have been documented elsewhere, highlighting the need for ongoing surveillance (2