6.2 Shunt Infections, CSF Correlates, Antigen and Molecular Methods

Shunt and Reservoir Infections Are Not Community Meningitis

Outline II.B lists other Staphylococcus spp., Corynebacterium spp., Propionibacterium spp., and Cutibacterium spp. as the indigenous organisms whose significance you must judge. In lumbar-puncture CSF from a patient without hardware, those organisms are usually skin contaminants. In ventriculoperitoneal shunts, ventriculoatrial shunts, external ventricular drains, and Ommaya reservoirs, they are the expected pathogens.

Staphylococcus epidermidis and other coagulase-negative staphylococci (CoNS) are the most common shunt-infection isolates. They adhere to silicone and plastic with biofilm: polysaccharide and protein matrix that shields organisms from neutrophils and from many antibiotics. Culture of CSF tapped from the reservoir may be positive while a lumbar puncture is still negative, because the infection is localized to the device. A single colony of CoNS from a community LP is often contamination; the same colony from a chronically febrile child with a VP shunt is a different conversation. Staphylococcus aureus still appears, usually with a more acute, purulent course and often with wound or tunnel infection.

Corynebacterium spp. (non-diphtheria coryneforms) and Cutibacterium acnes (formerly Propionibacterium acnes) are anaerobic or aerotolerant Gram-positive rods that colonize sebaceous skin. They infect shunts and other neurosurgical hardware weeks to months after implantation. Colonies may not appear in 24 hours. C. acnes often needs anaerobic media and 5–7 days. If the laboratory discards "diphtheroids" at 48 hours of aerobic incubation only, it will miss a real shunt pathogen. The outline still prints Propionibacterium spp. alongside Cutibacterium spp. because older reports and some item banks use the former name; treat them as the same clinical problem on M.

Biofilm has two laboratory consequences. First, Gram stain of device fluid can be positive when quantitative growth is modest. Second, device removal is often required for cure because antibiotics penetrate biofilm poorly—the microbiologist's job is still to recover and identify the organism, not to decide surgery, but the identification must not be watered down as "probable contaminant" in the report without the source being obvious.

Exam scenario. CSF from a VP-shunt tap grows white, catalase-positive, coagulase-negative colonies after 48 hours, and the cytospin showed rare Gram-positive cocci in clusters with few neutrophils. The correct exam move is to identify the CoNS and communicate the result as a shunt-associated isolate, not to reculture only the next LP because "CoNS is always skin flora."

Correlate Chemistry and Cell Count With the Smear

M asks you to correlate culture with CSF glucose, protein, and cell count. Use the pattern, not a single cutoff memorized without the serum glucose.

Low glucose in bacterial meningitis happens because organisms and neutrophils consume glucose and because transport at the choroid plexus is impaired. High protein reflects blood–brain barrier leakage. PMN pleocytosis is the acute bacterial fingerprint; lymphocytic pleocytosis is the viral fingerprint. Exceptions exist (early enterovirus can look neutrophilic for a day; Listeria can have a mixed or lymphocytic picture), which is why the Gram stain and culture still run even when the chemistry is "viral."

Do not diagnose bacterial meningitis from a high protein alone in a bloody traumatic tap; correct for serum values and look at the cytospin. Do not call a shunt infection "ruled out" because the glucose is normal.

Antigen Detection: Historical Latex, Limited Sensitivity

Older laboratories used latex agglutination on CSF supernatant for S. pneumoniae, H. influenzae type b, N. meningitidis (selected serogroups), GBS, and E. coli K1. The outline still lists antigen detection, so you must know what the test was and why it is no longer a stand-alone tool.

Sensitivity is limited, especially after antibiotics, with low organism load, and with meningococcal serogroups the kit does not cover. Specificity problems include rheumatoid-factor-like interference and capsule cross-reactions (K1 E. coli and meningococcal group B). A negative latex test never cancels a Gram stain or culture. A positive latex test does not identify a colony for susceptibility testing. Cryptococcal antigen remains clinically useful, but that is a mycology method, not a substitute for the bacterial latex kits described here.

If an item stem offers latex antigen as the next step after a negative cytospin in untreated pneumococcal disease, the better next steps are culture, broth, and molecular detection, not reliance on latex.

Molecular Methods: Multiplex Meningitis/Encephalitis PCR

Syndromic meningitis/encephalitis PCR panels (the commercial example most laboratories use is a multiplex cartridge on CSF) detect a fixed list of bacteria and viruses in about an hour. Bacterial targets typically include S. pneumoniae, N. meningitidis, H. influenzae, L. monocytogenes, S. agalactiae (GBS), and E. coli. Viral targets that the M outline cares about in this chapter are HSV and enterovirus; panels also commonly include VZV, CMV, HHV-6, and human parechovirus, plus Cryptococcus neoformans/gattii. That list is why a "viral-pattern" CSF still gets a molecular test: HSV encephalitis is treatable and enterovirus explains many lymphocytic meningitides in children.

Know the limits as tightly as the targets. PCR does not detect every bacterium that can sit in CSF: CoNS, C. acnes, Corynebacterium, S. aureus (on some versions), and many Enterobacteriaceae other than E. coli may be absent. A negative panel in a shunt patient therefore means nothing about biofilm infection. PCR does not produce an isolate for antimicrobial susceptibility testing. A pneumococcal PCR-positive, culture-negative specimen in a pretreated patient is useful for diagnosis and still leaves the physician without an MIC. False positives occur (HHV-6 latency in leukocytes is the usual teaching example). False negatives occur when the organism is off-panel or below the limit of detection.

Use PCR with Gram stain and culture, not instead of them. If the cytospin already shows lancet-shaped diplococci, you still plate CHOC and BAP even though the pneumococcal PCR is pending.

Critically Communicate the Gram Stain

A positive CSF Gram stain is a critical result. Call it as soon as it is read, before overnight growth, because empiric meningitis therapy is hours-sensitive. The report should name the morphology, not a species guess presented as fact: "Gram-negative diplococci" is the stain result; N. meningitidis is the leading interpretation in an adolescent, confirmed by culture or PCR. Document who was notified and the time. Positive cultures for the acute pathogens, and PCR positives for treatable targets such as HSV or meningococcus, follow the same critical-call pathway in most laboratories.

Shunt isolates deserve a phone call as well when the clinician may still think "contaminant." The outline's indigenous list is a trap for the candidate who auto-rejects CoNS. Communicate the source (shunt tap versus LP), the Gram morphology, and that biofilm organisms are significant in that context.

Exam scenario. CSF glucose 18 mg/dL, protein 220 mg/dL, WBC 2,400 with 90% neutrophils, cytospin with Gram-negative diplococci: call immediately, plate in CO2, add the multiplex panel if it is the laboratory's STAT menu, and do not wait for latex antigen to "confirm" meningococcus. Exam scenario. VP-shunt fluid with glucose 55 mg/dL, protein 60 mg/dL, WBC 80, rare Gram-positive rods on smear that grow anaerobically on day 5 as C. acnes: that is a shunt infection pattern, not a reason to change the report to "mixed skin flora, suggest reculture."