Fever With Rash
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Direct answer
Fever with rash is a diagnostic pattern in which the morphology, distribution and timing of the eruption narrow the differential sharply. Petechiae that do not blanch point to meningococcaemia, dengue or rickettsial disease and need urgent assessment, while a maculopapular rash spreading downward from the hairline with Koplik spots suggests measles. A rash that appears after a new drug course suggests a drug eruption.
What you must remember
- Measles: Cough, coryza and conjunctivitis with Koplik spots on the buccal mucosa; the maculopapular rash starts at the hairline and behind the ears, then spreads downward.
- Rubella: A pale pink, discrete rash starting on the face, with postauricular and suboccipital lymphadenopathy; congenital rubella is the feared complication in pregnancy.
- Dengue: A maculopapular rash, sometimes with petechiae, and the classic convalescent sign of white islands in a sea of red, after a critical phase of plasma leakage.
- Enteric fever: Rose spots, small salmon-pink macules on the trunk, usually appearing in the second week, with relative bradycardia.
- Scrub typhus and rickettsioses: An eschar at the bite site and a maculopapular rash that may turn petechial, treated with doxycycline.
- Meningococcaemia: Non-blanching petechiae or purpura, often on the lower limbs with meningism, needing emergency benzylpenicillin or ceftriaxone and chemoprophylaxis for contacts.
A graded approach to a febrile patient with a rash
Start with a glass test. Pressing a clear glass over the rash shows whether the petechiae blanch, and non-blanching lesions in a febrile patient are an emergency until meningococcal disease is excluded. The clinician then checks for meningism, drowsiness and hypotension, and gives parenteral antibiotics without waiting for the laboratory if the picture is toxic. If the rash is maculopapular and blanching, the next question is timing and exposure. A rash spreading from the hairline downward after three to four days of fever, with cough and conjunctivitis, is measles, and vaccination status matters. A febrile traveller with headache and a rash after a monsoon journey raises dengue, which needs platelet and haematocrit monitoring for plasma leakage. An eschar with a maculopapular rash after a rural exposure suggests scrub typhus, where doxycycline is started on suspicion. A rash after a new antibiotic course with eosinophilia is a drug eruption until proven otherwise, and mucosal involvement must be checked to exclude severe reactions. The examiner's favourite follow-up question is what to do next, and the answer is to name the most dangerous diagnosis first and then the simplest test that confirms or excludes it.
Where the exam tries to catch you
The commonest trap is confusing blanching with non-blanching rashes. Petechiae in a meningococcal picture do not fade under pressure, whereas the erythema of most viral exanthems does, so the glass test is more informative than the colour alone. Students also mix up measles and rubella: both are maculopapular, but measles has Koplik spots and high fever, while rubella is milder with prominent lymphadenopathy and matters chiefly in pregnancy. Rose spots are a classic enteric fever sign that should not be mistaken for a rickettsial rash, which has an eschar and different timing. Finally, a rash in a patient on a new drug with fever is not automatically a viral exanthem; eosinophilia and mucosal involvement point to a dangerous drug reaction.
Frequently asked questions
What is the glass test and why does it matter in fever with rash?
The glass test presses a clear glass tumbler on the rash to see whether the lesions fade. Non-blanching petechiae or purpura in a febrile patient suggest meningococcal disease or another emergency, and they need immediate antibiotics and specialist review.
Which rash is associated with Koplik spots?
Measles is the rash associated with Koplik spots, which are small white spots on the buccal mucosa opposite the molars. They appear before the rash, and their presence with cough, coryza and conjunctivitis is a classic clinical clue.
Why is dengue rash described as white islands in a sea of red?
The convalescent dengue rash shows areas of normal skin, the white islands, surrounded by red blanching erythema, which is what the phrase describes. This rash appears as fever settles and is distinct from the early petechial features of plasma leakage.
What rash appears in enteric fever and when?
Rose spots, small salmon-pink macules on the trunk, appear in enteric fever usually in the second week and fade over a few days. They are often accompanied by relative bradycardia and splenomegaly, which supports the diagnosis.