Systemic Lupus Erythematosus (SLE)
The most likely diagnosis is systemic lupus erythematosus (SLE), based on the constellation of multi-system involvement (arthritis, serositis, cytopenias) combined with a strongly positive ANA at 1:320, which exceeds the diagnostic threshold and makes SLE highly likely with 95.8% sensitivity and 86.2% specificity for systemic autoimmune rheumatic diseases. 1
Clinical Reasoning
Why SLE is the Correct Diagnosis
This patient meets multiple diagnostic features of SLE:
Positive ANA at 1:320 - This titer exceeds the 1:40 threshold that strongly argues against SLE when negative, and at >1:40 merits full SLE evaluation 2. The American College of Rheumatology indicates that ANA at 1:320 has optimal diagnostic performance for systemic autoimmune rheumatic diseases 1.
Characteristic cytopenias - The patient demonstrates the classic triad of SLE-associated cytopenias: anemia (Hb 89 g/L), leukopenia (WBC 3.2 x 10⁹/L), and thrombocytopenia (platelets 99 x 10⁹/L), all of which are characteristic laboratory findings in SLE 3, 1. These cytopenias have been associated with organ involvement, disease progression, and worse prognosis in SLE 3.
Serositis - The pleuritic chest pain with confirmed pleural effusion on CT represents serositis, a hallmark manifestation of SLE 3.
Inflammatory arthritis - Synovitis of small joints of the hands with arthralgia is consistent with SLE polyarthritis 3.
Constitutional symptoms - The 6-month history of fever, weight loss, and elevated inflammatory markers (CRP 99 mg/L, ESR 56 mm/h) fits the systemic inflammatory nature of SLE 3.
Why Adult-Onset Still's Disease (AOSD) is Unlikely
Several key features argue strongly against AOSD:
Ferritin level is too low - The ferritin of 920 pg/L, while elevated, is far below the typical range for AOSD, which characteristically presents with ferritin levels >1000 ng/mL 1. AOSD typically shows markedly elevated ferritin as a distinguishing feature 3.
Absence of characteristic rash - AOSD presents with a salmon-pink, evanescent, maculopapular rash predominantly on proximal limbs and trunk in 72.7% of cases 3. This patient has no rash, which is explicitly noted in the presentation 3.
Fever pattern is atypical - AOSD characteristically presents with quotidian (daily spiking) fever that accompanies the rash 3. The presentation does not describe this specific fever pattern 3.
CRP elevation pattern - While CRP is elevated here, in SLE patients rarely have significantly elevated CRP levels, and values >50 mg/L should prompt evaluation for superimposed infection 3. However, the CRP of 99 mg/L in this context with other SLE features is more consistent with active SLE than AOSD 3.
Why Infective Endocarditis is Unlikely
Normal heart sounds - The examination reveals normal heart sounds with no murmurs, making endocarditis unlikely 1.
No nail fold infarcts - The absence of peripheral stigmata of endocarditis (specifically noted as absent nail fold infarcts) argues against this diagnosis.
Positive ANA - While infections can occasionally produce positive ANA results 4, the strongly positive ANA at 1:320 combined with the specific pattern of cytopenias and serositis is much more consistent with SLE than infection-related autoantibodies 1, 2.
Recommended Next Steps
To confirm the SLE diagnosis, the following specific tests should be ordered:
Anti-dsDNA antibodies using Crithidia luciliae immunofluorescence test for highest specificity, as these are highly specific for SLE and correlate with disease activity and renal involvement 3, 1, 5.
Anti-Smith (Sm) antibodies, which are highly specific for SLE with strong confirmatory power 1, 5.
Urinalysis with microscopy to evaluate for lupus nephritis (proteinuria, cellular casts), as renal involvement provides critical prognostic information 3, 1.
Complement levels (C3, C4) as these are sometimes associated with active disease, particularly renal involvement 3.
Common pitfall to avoid: Do not dismiss SLE based on the elevated CRP alone. While many authors report that SLE patients rarely have elevated CRP, significant increases can occur and should prompt evaluation for superimposed infection only when CRP is very high (>50 mg/L) 3. The CRP of 99 mg/L here, while elevated, occurs in the context of clear multi-system SLE features and does not exclude the diagnosis 3.