How can I differentiate multisystem inflammatory syndrome in children (MIS-C), systemic juvenile idiopathic arthritis (Still's disease), macrophage activation syndrome (MAS), and infection?

Differentiating MIS-C, Still's Disease, MAS, and Infection

Start with SARS-CoV-2 testing and temporal association—if positive or recent exposure exists with multisystem inflammation, suspect MIS-C; if ferritin >10,000 ng/mL with cytopenias and hypofibrinogenemia, suspect MAS; if quotidian fevers with salmon-pink rash and arthritis without SARS-CoV-2 link, suspect Still's disease; always rule out active infection with cultures and imaging before diagnosing hyperinflammatory syndromes.

Key Diagnostic Algorithm

Step 1: Establish SARS-CoV-2 Association

• MIS-C requires temporal association with SARS-CoV-2 (positive PCR, antigen test, or elevated IgG antibodies) 1
• MIS-C typically occurs 2-6 weeks post-infection
• Critical point: Always evaluate for other infections that could explain the presentation 1

Step 2: Age and Demographics

• MIS-C: Broader age range (median ~8 years), increased in African, Afro-Caribbean, Hispanic descent 1
• Still's disease: Bimodal distribution—systemic JIA in younger children (median ~12 years), AOSD in adults 2
• MAS: Can complicate any of these conditions at any age 3

Step 3: Clinical Presentation Patterns

MIS-C distinguishing features 1, 4:

• Prominent GI symptoms (72.7% of cases—abdominal pain, diarrhea, vomiting)
• Neurologic involvement (22.1%—altered mental status, headache, encephalopathy)
• Shock presentation (more frequent than KD or Still's)
• Cardiac dysfunction: Arrhythmias, ventricular dysfunction, elevated troponin/BNP
• Conjunctivitis, mucosal changes, rash (overlaps with KD)

Still's disease distinguishing features 2:

• Quotidian (spiking) fever pattern: Daily temperature spikes to >39°C, returning to baseline
• Salmon-pink evanescent rash: Appears with fever, fades between spikes
• Arthritis/arthralgia: Prominent joint involvement
• Serositis (pericarditis, pleuritis)
• Hepatosplenomegaly, lymphadenopathy

MAS distinguishing features 5, 6, 7:

• Unremitting fever (continuous, not spiking)
• CNS involvement: Altered mental status, seizures (OR 4.30 for MAS) 8
• Hepatosplenomegaly
• Bleeding manifestations (DIC)
• Can complicate MIS-C, Still's disease, or infections

Step 4: Laboratory Differentiation (Most Critical)

Ferritin levels (key discriminator):

• MIS-C: Median ~440 ng/mL 4
• Still's disease: Elevated but typically <10,000 ng/mL
• MAS: Markedly elevated >10,000 ng/mL (median 17,863 ng/mL in sJIA-MAS) 6
• MAS in MIS-C: >469 μg/L suggests MAS complication 5

Platelet count (critical differentiator):

• MIS-C: Thrombocytopenia (median 173,000/μL) 4
• Still's disease: Often thrombocytosis initially
• MAS: Severe thrombocytopenia (<114 × 10⁹/L has 100% sensitivity for MAS in MIS-C) 5
• MAS in sJIA: Median 119 × 10⁹/L 7

Lymphocyte count:

• MIS-C: Lymphopenia (median 950-1140 cells/μL) 4, 6
• Still's disease: Leukocytosis with neutrophilia
• MAS: Leukopenia (OR 5.98 for MAS) 8

Fibrinogen:

• MIS-C: Normal or elevated
• Still's disease: Elevated
• MAS: Hypofibrinogenemia <2.5 g/L (OR 4.03 for MAS) 6, 8

Liver enzymes:

• MIS-C: Mild elevation
• Still's disease: Moderate elevation (AST/ALT 2-3× normal)
• MAS: Marked elevation (AST median 534-666 U/L, ALT 160-559 U/L) 7

Other key labs:

• LDH: Highest in MAS (median 890 U/L) vs MIS-C 6
• D-dimer: Elevated in MIS-C and MAS, but markedly elevated in MAS (median 16,800 ng/mL) 7
• Triglycerides: Hypertriglyceridemia specific for MAS 6, 7
• CRP: Highest in MIS-C (median 194 mg/dL) 6

Step 5: Cardiac Evaluation

• MIS-C: Elevated BNP/pro-BNP (median 1108 pg/mL), reduced LVEF, coronary artery abnormalities possible 4
• Still's disease: Pericarditis common, myocarditis less frequent
• MAS: Myocardial dysfunction can occur but less prominent than MIS-C

Step 6: Rule Out Infection

Critical step before diagnosing hyperinflammatory syndromes 1:

• Blood, urine, respiratory cultures
• Consider scrub typhus, dengue, leptospirosis in endemic areas 9
• Imaging: Chest X-ray, abdominal ultrasound, CNS imaging if indicated
• Lumbar puncture if neurologic symptoms present
• Infections can trigger MAS or coexist with MIS-C 9

Practical Clinical Approach

When to suspect MIS-C:

1. SARS-CoV-2 exposure/positive serology within 2-6 weeks
2. Multisystem involvement (especially GI + cardiac)
3. Shock or hypotension
4. Ferritin 400-1000 ng/mL with thrombocytopenia
5. Elevated inflammatory markers with lymphopenia

When to suspect Still's disease:

1. No SARS-CoV-2 association 2
2. Quotidian fever pattern >1-2 weeks
3. Salmon-pink evanescent rash
4. Prominent arthritis
5. Very high ferritin but <10,000 ng/mL
6. Thrombocytosis or normal platelets

When to suspect MAS (complicating any condition):

1. Ferritin >10,000 ng/mL (or >469 in MIS-C context) 5, 6
2. Platelets <114 × 10⁹/L 5
3. Hypofibrinogenemia <2.5 g/L 8
4. Cytopenias (especially leukopenia)
5. Marked transaminitis (AST/ALT >500 U/L)
6. Hypertriglyceridemia
7. CNS involvement
8. Unremitting fever despite treatment

When to suspect infection:

1. Positive cultures or PCR for specific pathogens
2. Focal findings (pneumonia, abscess, meningitis)
3. Response to antimicrobials
4. Always exclude before immunosuppression 1

Critical Pitfalls to Avoid

1. Don't assume MIS-C without SARS-CoV-2 evidence—test both PCR and serology 1
2. Don't miss MAS complicating MIS-C or Still's disease—monitor ferritin, platelets, fibrinogen serially 5, 7
3. Don't delay infection workup—obtain cultures before starting immunosuppression 1
4. Don't ignore CNS symptoms—strongly associated with MAS (OR 4.30) 8
5. Don't rely on single lab value—use constellation of findings 4, 6
6. Consider genetic testing for HLH-related variants in severe/recurrent cases—22.4% carry variants associated with worse outcomes 3

Admission Criteria (High Priority)

Immediate admission required for 1:

• Shock or hypotension
• Respiratory distress
• Neurologic changes (altered mental status, seizures)
• CRP >10 mg/dL with abnormal vitals
• Elevated troponin or BNP
• Suspected MAS (ferritin >10,000, thrombocytopenia, hypofibrinogenemia)

Multidisciplinary team essential: Rheumatology, cardiology, infectious disease, hematology, with ICU involvement for severe cases 1