Meningococcemia laboratory findings
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-In-Chief: Cafer Zorkun, M.D., Ph.D. [2]
Overview
Meningococci is usually identified from blood or CSF analysis. Aspiration or skin biopsy of the rash yield meningococci. CSF analysis usually shows increased protein, low glucose and increased number of neutrophils.
Laboratory Findings
- N. meningitidis is a gram-negative, encapsulated, aerobic diplococcus.
- Thirteen different meningococcal serologic groups have been defined, five of which cause the great majority of disease (A, B, C, Y, and W135).
- The distinction between serogroups is based on the immunochemistry of the capsular polysaccharide, but more recently PCR of capsule biosynthesis genes has been used for determining the serogroup of isolates.[1]
- Serogroup A, C, Y and W135 polysaccharides all elicit a serogroup-specific immune response, which allows for a successful quadrivalent vaccine.
- The serogroup B capsular polysaccharide is poorly immunogenic, thus making it challenging to develop a vaccine to protect against this serogroup.
- Vaccine development efforts for serogroup B are focusing on outer membrane proteins (OMPs) or other surface molecules rather than the capsular polysaccharide.
Identification of N. meningitidis
- The case definition for confirmed meningococcal disease requires isolation of N. meningitidis from a normally sterile site.
- Typically, the isolate comes from blood or CSF, but it can also be from joint, pleural, or pericardial fluid. Aspirates or skin biopsies of purpura or petechiae can yield meningococci in cases of meningococcemia.
- The typical medium used to grow the organism is chocolate agar or Mueller-Hinton medium in an atmosphere containing 5% carbon dioxide.
- Gram staining is commonly used for identification of N. meningitidis and continues to be a reliable and rapid method for presumptive identification. If proper quality assurance/quality control is performed, intracellular gram-negative diplococci in CSF can be considered meningococci until proven otherwise.
- In addition to bacteriology for definitive detection and identification, latex agglutination can be used for rapid detection of meningococcal capsular polysaccharides in CSF, although false-negative and false-positive results can occur. Antigen agglutination tests on serum or urine samples are unreliable for diagnosis of meningococcal disease.[2]
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- Real-time PCR detects DNA of meningococci in blood, CSF, or other clinical specimens.
- A major advantage of PCR is that it allows for detection of N. meningitidis from clinical samples in which the organism could not be detected by culture methods, such as when a patient has been treated with antibiotics before obtaining a clinical specimen for culture.
- Even when the organisms are nonviable following antimicrobial treatment, PCR can still detect N. meningitidis DNA.22 Because of the severity of meningococcal disease, it is critical to treat the patient as soon as infection is suspected, and not to delay to obtain culture or laboratory results first.
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Blood tests will be done to rule out other infections and help confirm meningococcemia. Such tests may include:
- Blood culture
- Complete blood count with differential
- Clotting studies (PT, PTT)
Other tests that may be done include:
- Lumbar puncture to obtain spinal fluid sample for CSF culture
- Skin biopsy and gram stain
- Urinalysis
- Invasive meningococcal disease is typically diagnosed by isolation of N. meningitidis from a normally sterile site. However, sensitivity of bacterial culture may be low, particularly when performed after initiation of antibiotic therapy. A Gram stain of cerebrospinal fluid showing gram-negative diplococci strongly suggests meningococcal meningitis.
- Kits to detect polysaccharide antigen in cerebrospinal fluid are rapid and specific, but false-negative results are common, particularly in serogroup B disease. Antigen tests of urine or serum are unreliable.
- Serologic testing (e.g., enzyme immunoassay) for antibodies to polysaccharide may be used as part of the evaluation if meningococcal disease is suspected but should not be used to establish the diagnosis.
Electrolyte and Biomarker Studies
- Blood cultures were positive in 41.4% of 400 cultures in one series of patients with meningococcal disease.
- Cerebrospinal fluid (CSF) culture and gram stain were positive in 94% of patients in the same series.[4]
- The ability to culture the meningococcus from CSF will not be substantially reduced if the cultures are obtained within one hour of antibiotic administration.
- One series revealed a median leukocyte count of approximately 1200 in CSF (range 10 to 65,000/mm). Approximately 75% had a CSF glucose level less than 40 mg/100 mL. CSF protein ranged from 25 to over 800 mg/100 mL with a median value of 150 mg/100 mL.
- Polymerase chain reaction (PCR) has been shown in a study with 54 samples to be 91% sensitive and specific, particularly useful when cultures may be negative due to prior antibiotic administration.[5]
- Latex agglutination can detect A, B, C, Y, and W-135 though the sensitivity of detecting serogroup B is low.
References
- ↑ Mothershed EA, Sacchi CT, Whitney AM, Barnett GA, Ajello GW, Schmink S; et al. (2004). "Use of real-time PCR to resolve slide agglutination discrepancies in serogroup identification of Neisseria meningitidis". J Clin Microbiol. 42 (1): 320–8. PMC 321732. PMID 14715772.
- ↑ Rosenstein NE, Perkins BA, Stephens DS, Popovic T, Hughes JM (2001). "Meningococcal disease". N Engl J Med. 344 (18): 1378–88. doi:10.1056/NEJM200105033441807. PMID 11333996.
- ↑ 3.0 3.1 3.2 3.3 "The Centers for Disease Control and Prevention(CDC)".
- ↑ Coovadia YM, Solwa Z (1987). "Three latex agglutination tests compared with gram staining for the detection of bacteria in cerebrospinal fluid". S Afr Med J. 71 (7): 442–4. PMID 2436320.
- ↑ Bryant PA, Li HY, Zaia A, Griffith J, Hogg G, Curtis N; et al. (2004). "Prospective study of a real-time PCR that is highly sensitive, specific, and clinically useful for diagnosis of meningococcal disease in children". J Clin Microbiol. 42 (7): 2919–25. doi:10.1128/JCM.42.7.2919-2925.2004. PMC 446275. PMID 15243039.