Tests
1st tests to order
CBC
Test
Platelet count below 150,000/microliter in 30% to 60% of patients.[14][15] Leukocyte count and hemoglobin 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 hospitalize, administer blood products).
Result
low platelet count; white cell count normal or slightly elevated with a shift toward immature leukocytes
serum electrolytes and blood urea nitrogen
Test
Serum sodium <135 mEq/dL in 20% to 50% of patients; BUN >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 hospitalize).
Result
low sodium; elevated BUN
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 hospitalize).
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 titer establishes probable diagnosis of RMSF. The diagnosis is confirmed by demonstrating a 4-fold or greater change in titer 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]
Serologic diagnosis does not differentiate between spotted fever group Rickettsia species in most laboratories.[1][20]
Result
titer ≥1:64
blood culture
Test
Should be obtained in all patients to rule out alternate diagnoses.
Result
normal
Tests to consider
complement fixation
Test
Alternate serology testing modality; preferred method is IFA.
Elevated titer suggests probable diagnosis of RMSF. Diagnosis is confirmed by demonstrating a 4-fold or greater change in titer 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]
Serologic diagnosis does not differentiate between spotted fever group Rickettsia species in most laboratories.[1][20]
Result
titer ≥1:16
latex agglutination
Test
Alternate serology testing modality; preferred method is IFA.
Elevated titer suggests probable diagnosis of RMSF. Diagnosis is confirmed by demonstrating a 4-fold or greater change in titer 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]
Serologic diagnosis does not differentiate between spotted fever group Rickettsia species in most laboratories.[1][20]
Result
titer ≥1:128
indirect hemagglutination
Test
Alternate serology testing modality; preferred method is IFA.
Elevated titer suggests probable diagnosis of RMSF. Diagnosis is confirmed by demonstrating a 4-fold or greater change in titer 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]
Serologic diagnosis does not differentiate between spotted fever group Rickettsia species in most laboratories.[1][20]
Result
titer ≥1:128
microagglutination
Test
Alternate serology testing modality; preferred method is IFA.
Elevated titer suggests probable diagnosis of RMSF. Diagnosis is confirmed by demonstrating a 4-fold or greater change in titer 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]
Serologic diagnosis does not differentiate between spotted fever group Rickettsia species in most laboratories.[1][20]
Result
titer ≥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 titer.[1]
Serologic 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 neurologic abnormalities: for example, mental status changes. Would typically find a mononuclear cell pleocytosis (<100 cells/microliter), 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]
Serologic diagnosis does not differentiate between spotted fever group Rickettsia species in most laboratories.[1][20]
Result
4-fold or greater change in titer 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 edema
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 specialized research laboratories and at the CDC.[1][19]
Result
demonstration of rickettsiae in skin samples
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