What is the diagnosis for a patient with fever, rash, and positive IgG?

Diagnosis: Past Dengue Infection with Current Febrile Illness

The positive IgG with negative NS1 and IgM indicates past dengue exposure rather than acute dengue infection, so the current fever and rash require evaluation for alternative diagnoses including other arboviral infections (particularly chikungunya), rickettsial diseases, or secondary dengue with atypical serologic response. 1

Serologic Interpretation

• NS1 negative and IgM negative with IgG positive indicates this is NOT acute primary dengue fever, as NS1 antigen is detectable from day 1-10 of acute infection and IgM becomes positive after the first week of illness 1
• The isolated IgG positivity suggests prior dengue exposure (weeks to years ago), which is common in endemic areas 1
• The normal platelet count (234) argues against active dengue hemorrhagic fever, where thrombocytopenia (platelet <150) is a cardinal feature 1, 2

Differential Diagnosis for Current Presentation

Most Likely: Chikungunya

• Fever with rash affecting antecubital area and face is highly consistent with chikungunya, which presents with acute febrile illness, severe polyarthralgia, and rash lasting 1-7 days 3
• Chikungunya has a 5-7 day incubation period (range 2-12 days) and is transmitted by the same mosquito vectors as dengue 3
• Critical diagnostic approach: PCR or viral culture is most effective during the first 5-7 days of symptoms, while IgM/IgG serology becomes positive from day 5-7 onwards 3

Alternative Consideration: Rickettsial Disease

• Fever with maculopapular rash can indicate Rocky Mountain spotted fever or other rickettsial infections, particularly if there is history of tick exposure 4
• Key distinguishing features: rickettsial rashes classically involve palms and soles (present in two-thirds of RMSF cases), and patients may have thrombocytopenia and elevated liver enzymes 4
• Serology is often negative early in illness; fourfold rise in antibody titer between acute and convalescent specimens (≥3 weeks apart) confirms diagnosis 4

Less Likely: Secondary Dengue with Atypical Response

• Secondary dengue infections can occasionally show atypical serologic patterns, but the normal platelet count strongly argues against active dengue 1, 2
• Dengue typically causes thrombocytopenia in 69.51% of cases, along with fever (100%), headache (94.75%), and rash (37.86%) 2

Recommended Diagnostic Workup

Immediate testing should include:

• Chikungunya PCR (if within first 5-7 days of symptoms) or chikungunya IgM/IgG serology (if beyond day 5) 3
• Complete blood count with differential to assess for leukopenia or thrombocytopenia 4
• Liver function tests (AST/ALT) to evaluate for hepatic involvement 4, 2
• Rickettsial serology if tick exposure history is present 4

Clinical Pitfalls to Avoid

• Do not assume active dengue based solely on positive IgG - this represents past infection, not current disease 1
• Do not dismiss chikungunya because dengue IgG is positive - co-circulation of both viruses is common in endemic areas 3
• Do not wait for serologic confirmation to initiate supportive care - most arboviral infections are self-limiting and require symptomatic management 3
• Monitor for warning signs including persistent vomiting, abdominal pain, mucosal bleeding, lethargy, or development of thrombocytopenia, which would indicate progression to severe disease 1

Management Approach

• Symptomatic treatment with acetaminophen for fever and discomfort (avoid NSAIDs due to bleeding risk) 5
• Adequate hydration and close monitoring for development of warning signs 1
• Reassess if symptoms worsen or new warning signs develop, particularly hypotension, narrow pulse pressure, or signs of plasma leakage 1