Systemic Lupus Erythematosus (SLE)
The most likely diagnosis is Systemic Lupus Erythematosus (C), based on the constellation of multi-system involvement with arthralgia, fever, weight loss, pleuritic chest pain, synovitis, cytopenias (anemia, leukopenia, thrombocytopenia), elevated inflammatory markers, and a strongly positive ANA at 1:320. 1
Diagnostic Reasoning
Why SLE is the Correct Diagnosis
The patient fulfills multiple diagnostic criteria for SLE with characteristic laboratory and clinical features:
Positive ANA at 1:320 exceeds the diagnostic threshold and makes SLE highly likely when combined with clinical features, with specificity of 86.2% and sensitivity of 95.8% for systemic autoimmune rheumatic diseases 1
Cytopenias are characteristic of SLE: The patient demonstrates anemia (Hb 89 g/L), leukopenia (WBC 3.2 x 10⁹/L), and thrombocytopenia (platelet count 99 x 10⁹/L), which are hallmark features 1, 2
Multi-system involvement affecting at least two organ systems (musculoskeletal with arthralgia and synovitis, pulmonary with pleuritic chest pain and pleural effusion) strongly suggests SLE 3
Serositis is evident with pleuritic chest pain and confirmed small left pleural effusion on CT, which is a classic SLE manifestation 2
Inflammatory markers show elevated CRP (99 mg/L) and ESR (56 mm/h), consistent with active systemic inflammation 2
Lymphopenia (17% vs normal 20-40%) is specifically associated with SLE and increases infection risk 2
Why Adult-Onset Still's Disease (AOSD) is Unlikely
Several key features argue strongly against AOSD:
Ferritin level of 920 pg/L is elevated but far below the typical range for AOSD, which characteristically presents with ferritin levels >1000 ng/mL and often >5000 ng/mL 1, 2
Absence of the characteristic salmon-pink, evanescent rash that occurs in 51-87% of AOSD patients and typically appears with fever spikes 2
No quotidian fever pattern (daily spiking fevers) that is pathognomonic for AOSD 2
Absence of sore throat (reported in 38-92% of AOSD cases) 2
The strongly positive ANA at 1:320 is atypical for AOSD, where ANAs are typically negative or low-titer 2
Why Infective Endocarditis is Unlikely
Clinical and laboratory features exclude endocarditis:
Normal heart sounds on examination with no murmurs 2
Absence of nail fold infarcts (splinter hemorrhages) which would suggest septic emboli 2
No evidence of vegetation or valvular abnormality mentioned in the clinical assessment 2
The pattern of cytopenias and positive ANA points to autoimmune disease rather than infection 1
Critical Diagnostic Pitfalls to Avoid
Do not rely solely on ferritin levels in this context: Ferritin is an acute phase reactant and can be falsely elevated in lupus cases, but the level here (920 pg/L) is insufficient for AOSD diagnosis 4, 1
Do not dismiss SLE based on absence of rash: The patient lacks the classic malar rash, but SLE should be suspected in any patient with unexplained disease involving two or more organ systems, even without typical cutaneous manifestations 3
Recognize that CRP elevation in SLE warrants attention: While many authors report that SLE patients rarely have significantly elevated CRP, values >50 mg/L should prompt evaluation for superimposed infection, though this patient's CRP of 99 mg/L in the context of active multi-system disease is consistent with lupus serositis 2
Next Diagnostic Steps Required
Further serological testing should include:
Anti-dsDNA antibodies using Crithidia luciliae immunofluorescence test for highest specificity 1
Anti-Smith (Sm) antibodies, which are highly specific for SLE with strong confirmatory power 1
Urinalysis with microscopy to evaluate for lupus nephritis (proteinuria, cellular casts), as renal involvement may be asymptomatic 2, 1
Complement levels (C3, C4) which are often low in active SLE 2