In a 25-year-old woman presenting with fatigue, myalgia, arthralgia, decreased appetite, and alopecia, which laboratory tests should be ordered?

Laboratory Testing for Suspected Systemic Lupus Erythematosus

This clinical presentation—a young woman with fatigue, myalgia, arthralgia, decreased appetite, and alopecia—strongly suggests systemic lupus erythematosus (SLE) and requires immediate autoimmune serologic testing alongside basic screening laboratories.

Initial Essential Laboratory Tests

Autoimmune Panel (Priority Testing)

• Antinuclear antibody (ANA) - the primary screening test for SLE
• Anti-double stranded DNA (anti-dsDNA) antibodies - highly specific for SLE
• Anti-Smith (anti-Sm) antibodies - highly specific for SLE
• Complement levels (C3, C4) - typically decreased in active SLE
• Inflammatory markers:
  • Erythrocyte sedimentation rate (ESR)
  • C-reactive protein (CRP)

Complete Blood Count with Differential

Look specifically for:

• Anemia (chronic disease or hemolytic)
• Leukopenia (common in SLE)
• Lymphopenia (characteristic of SLE)
• Thrombocytopenia

Comprehensive Metabolic Panel

• Renal function (BUN, creatinine) - to assess for lupus nephritis
• Liver function tests (AST, ALT) - can be elevated in autoimmune hepatitis or myositis
• Electrolytes

Urinalysis with Microscopy

• Proteinuria (indicator of lupus nephritis)
• Hematuria
• Cellular casts

Secondary Testing Based on Clinical Context

Muscle Enzyme Testing (if myalgia is prominent)

• Creatine kinase (CK) - to evaluate for inflammatory myositis 1
• Aldolase - additional marker for muscle inflammation 1
• Lactate dehydrogenase (LDH) - can be elevated in myositis 1

The distinction here is critical: myalgia with pain but normal CK suggests polymyalgia-like syndrome or fibromyalgia, while elevated CK with weakness indicates true inflammatory myositis 1.

Additional Autoantibodies (if initial testing suggests specific complications)

• Antiphospholipid antibodies (lupus anticoagulant, anticardiolipin, anti-β2 glycoprotein I) - if thrombotic history
• Anti-Ro/SSA and anti-La/SSB - if sicca symptoms present
• Rheumatoid factor (RF) and anti-CCP - to differentiate from rheumatoid arthritis if prominent joint involvement 1

Thyroid Function Tests

• TSH and free T4 - hypothyroidism can present with fatigue, myalgia, arthralgia, and hair loss 2, 3
• This is essential as thyroid disease is a common mimic and can coexist with autoimmune conditions

Important Clinical Caveats

Do not assume a single diagnosis. While this constellation strongly suggests SLE, several conditions can present similarly:

1. Hypothyroidism mimics many of these symptoms and requires TSH screening 2, 3
2. Vitamin D deficiency can cause fatigue and musculoskeletal pain, though this is less likely to explain the full syndrome 4
3. Inflammatory myopathies (dermatomyositis/polymyositis) present with myalgia, fatigue, and can have alopecia 5

The presence of alopecia in this context is particularly significant - it suggests active systemic inflammation rather than isolated musculoskeletal complaints, making autoimmune screening imperative.

Algorithmic Approach

1. Order simultaneously: ANA, CBC with differential, CMP, urinalysis, ESR/CRP, TSH
2. If ANA positive: Reflex to anti-dsDNA, anti-Sm, complement levels (C3, C4)
3. If prominent myalgia: Add CK, aldolase, LDH
4. If joint swelling present: Add RF, anti-CCP 1
5. If urinalysis abnormal: Quantify with 24-hour urine protein or spot protein/creatinine ratio

The goal is rapid identification of SLE or other serious autoimmune disease, as early treatment significantly impacts morbidity and mortality. Delayed diagnosis of lupus nephritis, for example, can lead to irreversible renal damage.