Acute febrile rash (exanthem): when are tests needed? Measles, Kawasaki disease, drug eruption, primary HIV infection

• Notify the ARS (regional health agency) without delay (telephone, email) as soon as clinically suspected, then complete the mandatory notification form, without waiting for the result.

Laboratory confirmation

• Measles virus RNA by RT-PCR on an oropharyngeal sample (preferably in the first days of the rash; state the date of rash onset, contact and vaccination status)
• Measles serology IgM and IgG (from day 3 of the rash; or salivary IgM with the kit supplied by the ARS)if neededSerum or salivary IgM testing is possible from day 3 after the start of the rash, with oropharyngeal RT-PCR remaining preferred ^[2]. An IgM result is meaningful only in the absence of vaccination in the previous 2 months ^[3]. The NABM provides for serology in addition to PCR in a patient already immune (previous measles or vaccine) ^[4].

• Fever for 5 days or more with suggestive signs (even if they have disappeared): refer to the paediatric emergency department the same day, without prior tests in the community.
• The tests (FBC, CRP, albumin, transaminases, urine tests, echocardiography) are done in hospital.

• Skin detachment, blisters, mucosal erosions (eyes, mouth, genitals), fever > 39 °C with impaired general condition: stop the suspected drug and refer to the emergency department, without tests in the community (epidermal necrolysis).

Suspected DRESS (fever, facial oedema, lymphadenopathy): stop the drug and arrange hospital admission the same day; if these tests are done in the community, they must not delay admission

• FBC with blood film examination (eosinophils, atypical lymphocytes)
• Full liver function tests: AST
• ALT
• GGT
• Alkaline phosphatase
• Bilirubin
• Serum electrolytes
• Urea
• Serum creatinine with eGFR
• PT (prothrombin time)

Urine tests

• Urine dipstick (proteinuria, haematuria)
• Urine protein:creatinine ratio (if dipstick positive for protein)if needednon-French guidelineDRESS involves visceral damage in 91% of cases ^[14]; proteinuria on dipstick must be quantified (see the page “Proteinuria”).

Laboratory tests

• HIV serology (combined screening test)
• HIV-1 viral load (plasma RNA): acute infectious syndrome compatible with primary infectionif neededThe NABM covers HIV-1 viral load in adults and adolescents with any acute infectious syndrome compatible with HIV primary infection; HIV-2 primary infections are exceptional in France ^[4].
• Syphilis serology (depending on exposure)if neededRisky sexual exposure: see the pages “Sexually transmitted infections” and “Syphilis: positive serology” for the full tests.

• No laboratory tests: assess severity (child's fever, purpura) and review if the rash changes or if the fever lasts 5 days or more.