Investigations
1st investigations to order
saliva PCR and viral culture
Test
PCR detects rabies virus RNA.
The virus is more likely to be detectable at the beginning of the clinical course. Most patients with rabies have a positive result with repeated testing, as PCR testing is very sensitive.
Saliva, CSF, skin, and serum should be tested simultaneously for accurate diagnosis. A negative result in saliva alone does not rule out rabies. Results in saliva, skin biopsy, CSF, and serum (in non-immunised patients) must all be negative.
A positive saliva PCR or culture is also an indication of the infectivity of patient and the need for patient isolation and prevention measures in healthcare personnel.
If the patient survives long enough, this test becomes negative, indicating that the patient is no longer infectious.
Result
detection of rabies RNA; isolation of rabies virus
skin biopsy (neck) with direct fluorescent antibody (DFA) and PCR
Test
A 5- to 6-mm diameter section is taken from the posterior region of the neck at the hairline. A good specimen contains a minimum of 10 hair follicles and is taken deep enough to include the cutaneous nerves found at the base of hair follicles.
DFA testing detects rabies antigens in the sample.
PCR detects rabies virus RNA.
The virus is more likely to be detectable at the beginning of the clinical course.
A negative result in skin biopsy alone does not rule out rabies. Results in saliva, skin biopsy, CSF, and serum (in non-immunised patients) must all be negative.
Result
detection of rabies virus antigen by DFA; detection of rabies RNA
CSF cytology
Test
Detectable in 60% of patients in the first week and 85% of patients in the second week.
Result
mild lymphocytic pleocytosis
CSF biochemistry
Test
CSF protein levels may be mildly elevated (>0.5 g/dL [>50 mg/dL])), with a low to normal glucose concentration. CSF quinolinic acid is high early on, and lactate progressively increases over days.
Result
elevated protein; low or normal glucose; high quinolinic acid; high lactate
CSF rabies neutralising antibody
Test
Indicates rabies infection.
A negative result in CSF alone does not rule out rabies. Results in saliva, skin biopsy, CSF, and serum (in non-immunised patients) must all be negative.
Result
positive
serum rabies IgM or IgG
Test
Antibodies may not be detected at early stages of the disease; serial tests will demonstrate increase in antibody levels by day 14 of hospitalisation. Previously immunised patients will have rabies antibodies, and this test cannot be used to detect infection in these patients.
A negative result in serum alone does not rule out rabies. Results in saliva, skin biopsy, CSF, and serum (in non-immunised patients) must all be negative.
Result
positive
CSF herpes simplex PCR
Test
Used to exclude herpes simplex encephalitis.
Result
negative
CSF enterovirus PCR
Test
Used to exclude enterovirus meningoencephalitis.
Result
negative
CSF West Nile virus IgM
Test
Used to exclude West Nile virus encephalitis.
Result
negative
serum N-methyl-D-aspartate (NMDA) glutamate receptor antibodies
Test
Used to exclude common cause of limbic encephalitis.
Result
negative
Investigations to consider
serum arbovirus antibodies
Test
Used to exclude arbovirus encephalitides.
Result
negative
serum Bartonella antibodies
Test
Used to exclude Bartonella infection.
Result
negative
serum Rickettsia antibodies
Test
Used to exclude rickettsial infection.
Result
negative
head MRI
Test
Used to exclude acute disseminated encephalitis.
Result
normal in the first week of infection
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