Typhus laboratory findings

Diagnosis of typhus fever is usually based on clinical recognition and serology the latter requires comparison of acute- to convalescent-phase serology, so is only helpful in retrospect. Etiologic agents can generally only be identified to the genus level by serologic testing. PCR and immunohistochemical analyses may also be helpful.^[1][2][3]

A complete blood count (CBC) may show anemia and low platelets. Other blood tests for typhus may show:

• High level of typhus antibodies
• Low level of albumin
• Low sodium level
• Mild kidney failure
• Mildly high liver enzymes

• The indirect immunofluorescence assay (IFA) is generally considered the reference standard but is usually not available in developing countries where this disease is endemic.
• Other serological tests include ELISA and indirect immunoperoxidase (IIP) assays. Weil-Felix OX-K agglutination assays may be used in some international settings but lack sensitivity and specificity and are not generally used in the United States.
• These assays can detect either IgG or IgM antibodies.
• Diagnosis is typically confirmed by documenting a four-fold rise in antibody titer between acute and convalescent samples.
• Acute specimens are taken during the first week of illness and convalescent samples are taken 2–4 weeks later.
• IgG antibodies are considered more accurate than IgM, but detectable levels of IgG antibody generally do not appear until 7–10 days after the onset of illness.
• The most rapid and specific diagnostic assays for scrub typhus rely on molecular methods like polymerase chain reaction (PCR), which can detect DNA in a whole blood, eschar swab, or tissue sample
• Rickettsia typhi can be detected via indirect immunofluorescence antibody (IFA) assay, immunohistochemistry (IHC), polymerase chain reaction (PCR) assays using blood, plasma, or tissue samples, or culture isolation. PCR is most sensitive on samples taken during the first week of illness, but prior to the start of doxycycline.

A diagnosis of rickettsial diseases is based on two or more of the following:

• Clinical symptoms and an epidemiologic history compatible with a rickettsial disease
• Development of specific convalescent-phase antibodies reactive with a given pathogen or antigenic group
• Positive polymerase chain reaction test result
• Specific immunohistologic detection of rickettsial agent
• Isolation of a rickettsial agent. Ascertaining the likely place and the nature of potential exposures is particularly helpful for accurate diagnostic testing

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