eWe describe the use of PCR and electrospray ionization followed by mass spectrometry (PCR/ESI-MS) to evaluate "culture-negative" cerebrospinal fluid (CSF) from a 67-year-old man who developed postoperative bacterial ventriculitis following a suboccipital craniotomy for resection of an ependymoma in the 4th ventricle. CSF samples were obtained on seven occasions, beginning in the operating room at the time of insertion of a right ventriculoperitoneal shunt (VPS) and continuing until his death, 6 weeks later. During the course of the illness, two initial CSF specimens taken before the initiation of antimicrobial treatment were notable for growth of Enterococcus faecalis. Once antimicrobial treatment was initiated, all CSF cultures were negative. PCR/ESI-MS detected genetic evidence of E. faecalis in all CSF samples, but the level of detection (LOD) decreased once antimicrobial treatment was initiated. When our patient returned with symptoms of meningitis 3 days after the completion of antibiotic treatment, CSF cultures remained negative, but PCR/ESI-MS again found genetic evidence for E. faecalis at levels comparable to the pretreatment levels seen initially. This unique case and these findings suggest that determination of CSF LOD by PCR/ESI-MS may be a very sensitive indicator of persistent infection in patients on antibiotic therapy for complex CNS infections and may have relevance for treatment duration and assessment of persistent or recurrent infection at the completion of therapy. CASE REPORTA 67-year-old non-insulin-dependent diabetic man with a history of hypertension and coronary artery disease presented for evaluation of ataxia. He complained of approximately 3 months of "listing to the right upon walking"; this symptom was associated with gradually worsening headaches, nausea and vomiting, vertigo, and tremors. Other than tremor and a positive Romberg sign, results of the patient's examination were unremarkable. The examination showed that motor, sensory, and cranial nerves were intact. A mass consistent with an ependymoma was discovered in the 4th ventricle on magnetic resonance imaging (MRI) of the brain. Due to the significant mass effect and suspicion of malignancy, resection was indicated, and he was scheduled for resection of the 4th ventricle mass.A suboccipital craniotomy was performed. Preoperative cefazolin antibiotic prophylaxis was administered. After tumor resection, the dura was closed. Pathological examination of the fourth ventricular tumor was consistent with a mixed ependymoma/subependymoma, World Health Organization grade II. The morning following surgery, diplopia developed. Postoperative MRI of the brain demonstrated complete resection without complication. A pseudomeningocele was noted in the soft tissues, although no CSF leak was noted at the wound; a computed tomography (CT) scan of the head 24 h postoperation demonstrated hydrocephalus, prompting insertion of a right frontal external ventricular drain (EVD). The CSF appeared normal. At this time, CSF was not submitted for cult...