Investigations
1st investigations to order
FBC
Test
During the first phase, leukopenia and thrombocytopenia are usually found.
During the second phase, an elevated WBC is common.
Result
low WBC, low platelet count (first phase); elevated WBC (second phase)
erythrocyte sedimentation rate
Test
Likely to lie within the normal range.[4]
Result
normal
CRP
Test
Likely to lie within the normal range.[4]
Result
normal
LFTs
Test
An elevated result may occur in viral encephalitis.
Result
elevated
CT brain
Test
It should be ordered in all patients with altered mental state. Post-contrast CT scans (if obtained) can demonstrate diffuse meningeal enhancement, which is a frequent co-occurrence in encephalitis.
If there is no contraindication from CT scan, lumbar puncture can be performed.
Result
frequently normal early in the course of encephalitis, but may see more prominent changes
cerebrospinal fluid analysis
Test
Lumbar puncture (LP) is almost mandatory in all cases of suspected encephalitis provided that a mass lesion in the brain has been ruled out by imaging and no contraindications exist. LP should be delayed only under abnormal circumstances.
Result
may show elevated WBC, normal/elevated protein, normal/low glucose, normal/elevated RBC
cerebrospinal fluid/serum serology
Test
Acute-phase specimens should be tested for virus-specific IgM antibody.[20] IgM in cerebrospinal fluid is more specific for diagnosing encephalitis than IgM in serum.[20] IgM antibody is detectable within the first few weeks after disease onset, and persists for several months.[20] IgG antibody is generally detectable shortly after IgM and persists for years.[20]
Result
positive for IgM/IgG antibodies
Investigations to consider
cerebrospinal fluid/serum reverse-transcription polymerase chain reaction
Test
In the first few days of illness, serum or cerebrospinal fluid can be tested by reverse-transcription polymerase chain reaction for viral nucleic acids. However, TBE-infected people usually have only low levels of transient viraemia and have neutralising antibodies by the time they have clinically apparent infection; therefore, molecular testing for viral nucleic acids is less sensitive than serology.[20][29]
Result
may be positive
MRI head
Test
Abnormalities are seen in up to 20% of infected patients.[2]
Result
may show increased signal intensity on T2-weighted and fluid-attenuated inversion recovery images in the thalamus, basal ganglia, internal capsule, splenium, cerebellum, peduncles, and brainstem. The most common site for lesions is the thalamus. Lesions may be unilateral or bilateral; multifocal or diffuse
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