Investigations
1st investigations to order
FBC
Test
Platelet count <150,000/microlitre in 30% to 60% of patients.[14][15] Leukocyte count and haemoglobin are usually normal.
Ordered when diagnosis first suspected; normal results do not rule out Rocky Mountain spotted fever (RMSF), but markedly abnormal results (e.g., severe thrombocytopenia) may impact on patient management decisions (e.g., decision to hospitalise, administer blood products).
Result
low platelet count; white cell count normal or slightly elevated with a shift towards immature leukocytes
serum electrolytes and urea
Test
Serum sodium <135 mmol/L (135 mEq/dL) in 20% to 50% of patients; urea >9 mmol/L (25 mg/dL) in approximately 10% of patients.[14][15]
Ordered when diagnosis first suspected; normal results do not rule out RMSF, but markedly abnormal results (e.g., renal failure) may impact on patient management decisions (e.g., decision to hospitalise).
Result
low sodium; elevated urea
LFTs
Test
Most patients have mild elevations of AST and ALT (in children, median values of 83 units/L and 55 units/L, respectively).[14]
Ordered when diagnosis first suspected; normal results do not rule out RMSF, but markedly abnormal results (e.g., severe hepatitis) may impact on patient management decisions (e.g., decision to hospitalise).
Result
mildly elevated aminotransferases
indirect immunofluorescent antibody
Test
Indirect immunofluorescent antibody (IFA) from paired acute and convalescent serum is the reference standard.[1][19]
Preferred serology testing method. Elevated acute titre establishes probable diagnosis of RMSF. The diagnosis is confirmed by demonstrating a 4-fold or greater change in titre between acute-phase and convalescent-phase serum specimens.
Rarely shows positive results until at least 7 to 10 days into illness, but establishes baseline for subsequent confirmatory testing.[1]
Serological diagnosis does not differentiate between spotted fever group Rickettsia species in most laboratories.[1][20]
Result
titre ≥1:64
blood culture
Test
Should be obtained in all patients to rule out alternate diagnoses.
Result
normal
Investigations to consider
complement fixation
Test
Alternative serology testing modality; preferred method is IFA.
Elevated titre suggests probable diagnosis of RMSF. Diagnosis is confirmed by demonstrating a 4-fold or greater change in titre between acute-phase and convalescent-phase serum specimens.
Rarely shows positive results until at least 7 to 10 days into illness, but establishes baseline for subsequent confirmatory testing.[1]
Serological diagnosis does not differentiate between spotted fever group Rickettsia species in most laboratories.[1][20]
Result
titre ≥1:16
latex agglutination
Test
Alternative serology testing modality; preferred method is IFA.
Elevated titre suggests probable diagnosis of RMSF. Diagnosis is confirmed by demonstrating a 4-fold or greater change in titre between acute-phase and convalescent-phase serum specimens.
Rarely shows positive results until at least 7 to 10 days into illness, but establishes baseline for subsequent confirmatory testing.[1]
Serological diagnosis does not differentiate between spotted fever group Rickettsia species in most laboratories.[1][20]
Result
titre ≥1:128
indirect haemagglutination
Test
Alternative serology testing modality; preferred method is IFA.
Elevated titre suggests probable diagnosis of RMSF. Diagnosis is confirmed by demonstrating a 4-fold or greater change in titre between acute-phase and convalescent-phase serum specimens.
Rarely shows positive results until at least 7 to 10 days into illness, but establishes baseline for subsequent confirmatory testing.[1]
Serological diagnosis does not differentiate between spotted fever group Rickettsia species in most laboratories.[1][20]
Result
titre ≥1:128
microagglutination
Test
Alternative serology testing modality; preferred method is IFA.
Elevated titre suggests probable diagnosis of RMSF. Diagnosis is confirmed by demonstrating a 4-fold or greater change in titre between acute-phase and convalescent-phase serum specimens.
Rarely shows positive results until at least 7 to 10 days into illness, but establishes baseline for subsequent confirmatory testing.[1]
Serological diagnosis does not differentiate between spotted fever group Rickettsia species in most laboratories.[1][20]
Result
titre ≥1:128
enzyme immunoassay
Test
Alternate serology testing modality; preferred method is IFA.
Positive result suggests probable diagnosis of RMSF. These tests are qualitative, not quantitative, and thus are not helpful for assessing change in antibody titre.[1]
Serological diagnosis does not differentiate between spotted fever group Rickettsia species in most laboratories.[1][20]
Result
antibody detected
lumbar puncture
Test
Indicated if presentation includes fever and neurological abnormalities: for example, mental status changes. Would typically find a mononuclear cell pleocytosis (<100 cells/microlitre), an elevated protein concentration, and a normal glucose concentration.
Result
abnormal
convalescent serology
Test
Obtained 2 to 4 weeks after acute serum collected. Seroconversion retrospectively confirms the diagnosis.[1]
Serological diagnosis does not differentiate between spotted fever group Rickettsia species in most laboratories.[1][20]
Result
4-fold or greater change in titre between acute-phase and convalescent-phase serum specimens
chest x-ray
Test
Useful in a suspected case of RSMF infection.
Result
normal; may show pulmonary oedema
abdominal CT scan
Test
Useful to exclude other diagnoses such as appendicitis.
Result
normal
immunohistochemistry
Test
Can confirm the diagnosis early in the illness course by demonstrating the presence of rickettsiae in skin biopsy samples. This technique has a reported sensitivity of about 70% and specificity of 100%, but is time consuming and available only in specialised research laboratories and at the CDC.[1][19]
Result
demonstration of rickettsiae in skin samples
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