Effect of delayed lumbar punctures on the diagnosis of acute bacterial meningitis in adults

Introduction Bacterial meningitis is a medical emergency, the outcome of which is improved by prompt antibiotic treatment. For patients with suspected meningitis and no features of severe disease, the British Infection Society recommends immediate lumbar puncture (LP) before antibiotics, to maximise the chance of a positive cerebrospinal (CSF) culture. In such patients, CT scanning before LP is not needed. Methods The case notes of adults with meningitis admitted to a large district general hospital over 3 years were reviewed. Patients were classified as Likely Bacterial Meningitis or Likely Viral Meningitis based on their CSF and peripheral blood results using the Meningitest Criteria, with microbiological and virological confirmation. Results Of 92 patients studied, 24 had Likely Bacterial Meningitis, including 16 with microbiologically confirmed disease (none had PCR tests for bacteria). Sixty-eight had Likely Viral Meningitis, four of whom had viral PCR, including one with herpes simplex virus. No patient had an LP before antibiotics. CSF culture was positive for eight (73%) of the 11 patients who had an LP up to 4 h after starting antibiotics, compared with eight (11%) of 71 patients with a later LP (p<0.001). None of the 34 LPs performed more than 8 h after antibiotics was culture-positive. For 62 (67%) of the 92 patients, the delay was due to a CT scan, although only 20 of these patients had a contraindication to an immediate LP. Conclusions Too many patients with acute bacterial meningitis are being sent for unnecessary CT scans, causing delays in the LP, and reducing the chances of a positive CSF culture after starting antibiotics. However, even if antibiotics have been started, an LP within 4 h is still likely to be positive. Molecular tests for diagnosis should also be requested.