Differentials
Rubella
SIGNS / SYMPTOMS
Typically a mild illness with patient and rash improving in about 3 days.
INVESTIGATIONS
Rubella-specific IgM antibody or other specific test for rubella.
Scarlet fever
SIGNS / SYMPTOMS
Pharyngitis, anterior cervical adenopathy, and fever precede characteristic scarlatiniform rash (diffuse, finely papular, 'sandpaper-like', erythematous rash that blanches with pressure). Rash begins on the trunk and may spread; it is accentuated in flexural creases (i.e., under the arm, in the groin, in the elbows) often with petechiae, producing red streaks known as Pastia's lines. Cheeks flushed 'scarlet' with circumoral pallor. Skin desquamation may occur 3-4 days after appearance of the scarlatiniform rash. Tongue may be inflamed with a white coating and prominent papillae ('strawberry tongue').
INVESTIGATIONS
Group A streptococcal rapid antigen test; polymerase chain reaction test, or throat/skin swab culture. Diagnosis is often clinical.
Parvovirus B19
SIGNS / SYMPTOMS
Typically a mild illness, associated classically with rash initially presenting with "slapped cheek" appearance, followed by lacy, reticular lesions; may have associated arthritis and/or anemia. Fever has usually subsided before rash develops. Occurs much more commonly in the US than measles.
INVESTIGATIONS
Parvovirus specific antibody detection.
Roseola
SIGNS / SYMPTOMS
Rash appears after fever breaks.
INVESTIGATIONS
Specific test for human herpes virus 6 or human herpes virus 7.
Dengue
SIGNS / SYMPTOMS
Febrile illness in which headache and body aches are prominent. Rash may be petechial. Appropriate exposure including exposure to mosquitoes that may carry dengue.
INVESTIGATIONS
Specific test for dengue antibody or antigen.
EBV infection (mononucleosis)
SIGNS / SYMPTOMS
Febrile illness that may involve sore throat, pharyngeal exudates, and hepatosplenomegaly.
INVESTIGATIONS
Specific test for EBV antibody. Monospot for children over age 4 years.
Zika virus infection
SIGNS / SYMPTOMS
Mild febrile illness with itchy rash, headache, joint pain, and conjunctivitis. Appropriate exposure including exposure to mosquitoes that may carry Zika virus.
INVESTIGATIONS
Specific test for Zika antibody.
Drug eruptions
SIGNS / SYMPTOMS
History of recent medication or drug ingestion.
INVESTIGATIONS
No differentiating tests.
Coronavirus disease 2019 (COVID-19)
SIGNS / SYMPTOMS
Residence in/travel to a country/area or territory with local transmission, or close contact with a confirmed or probable case of COVID-19, in the 14 days prior to symptom onset.
The situation is evolving rapidly; see our COVID-19 topic for further information.
INVESTIGATIONS
Real-time reverse transcription polymerase chain reaction (RT-PCR): positive for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) RNA.
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