Tests
1st tests to order
CBC
Test
Leukocytosis may be the first indication of an invasive bacterial infection.
Thrombocytopenia may indicate impending disseminated intravascular coagulation associated with sepsis.
Result
elevated WBC count, low platelets
urine pregnancy test
Test
Listeriosis can present in all trimesters of pregnancy. Exclusion of pregnancy may be indicated in women of childbearing age.
Result
may be positive
blood cultures
Test
Ordered if high suspicion of bacteremia exists (positive epidemiologic risk factors) or with the presence of fever and rigors. Bacteremias are discovered in central nervous system (CNS) infections in over 50%, which significantly increases in patients with brain abscess.[1] Much lower sensitivity if prior antibiotics have been administered.
Result
positive
brain MRI
brain CT
cerebrospinal fluid (CSF) analysis
Test
CSF testing is done in patients presenting with signs and symptoms of CNS infection, generally following head imaging. Typically, the WBC count of the CSF is usually <5000 cells/mm³ and protein concentration is <200 mg/dL. During early CNS infection, CSF may be normal, and, if so, lumbar puncture should be repeated in 12-24 hours.
Gram stain of CSF is important for diagnosis in up to 75% of patients withListeria meningitis patients.[1][5] Due to the resemblance of the pathogen to diphtheroids or pneumococci and its tendency to become overdecolorized, the specificity of Gram stain is reduced.
Gram-positive organism on culture is the definitive test for diagnosis. CSF cultures are positive in 20% to 40% of CNS infection cases.[1][6][7][32]
Result
abnormal WBC count; positive Gram stain and culture
Tests to consider
prothrombin time (PT) and PTT
Test
Bleeding diathesis associated with disseminated intravascular coagulation may occur.
Result
may be abnormal
D-dimer
Test
Bleeding diathesis associated with disseminated intravascular coagulation may occur.
Result
may be elevated
placenta and amniotic fluid culture
Test
Ordered when there is neonatal infection or maternal signs and symptoms of invasive disease.
Result
positive
cervical swab culture
Test
May be done in addition to placental, amniotic fluid, and stool cultures to test for perinatal listeriosis.
Result
positive
meconium Gram stain and culture
Listeria serology
Test
Ordered when an outbreak is suspected. Low specificity due to cross-reactivity to nonpathogenic Listeria and other gram-positive organisms. May be negative in early infection.
Result
may be positive
food analysis
Test
Ordered when an outbreak is suspected or in severe sporadic infections.
Result
may be positive
stool culture
Test
May be done in outbreaks or severe sporadic infections. May be done in patients with gastroenteritis and in healthy persons during times of outbreaks. Healthy carriers have been known to comprise 5% of the population.[1] Among patients with Listeria gastroenteritis, stool cultures may be positive in up to 87%.[1]
Result
positive for Listeria; negative for other organisms
polymerase chain reaction of blood
Test
May be ordered when there is high suspicion of listeriosis and cultures are negative. Blood cultures may be negative after antibiotic treatment, reducing their sensitivity.
Result
may be positive
other stool analyses
Test
Stool for ova and parasites, and enzyme-linked immunosorbent assay or agglutination tests, will typically be negative for other pathogens such as Campylobacter jejuni or Cryptosporidium.
Result
negative for other organisms
electroencephalogram
Test
Seizures may occur late in the course of CNS infection.
Result
may be positive for seizure activity
echocardiography
Test
Endocarditis occurs more commonly in the immunocompromised population. May be performed in patients with evidence of bacteremia or murmur.
Result
may show vegetations
Emerging tests
meningitis/encephalitis panel
Test
Automated PCR multiplex testing of CSF for common pathogens, including Listeria monocytogenes, that cause central nervous system infections.
Result
may be positive
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