Systemic Lupus Erythematosus (SLE)

01Pathophysiology

SLE is a chronic, relapsing-remitting autoimmune disease driven by a fundamental breakdown in immune tolerance to self-antigens. The immune system fails to clear apoptotic cell debris, and nuclear material (DNA, histones, ribonucleoproteins) is exposed to the adaptive immune system. This triggers the production of a wide array of antinuclear antibodies (ANA), including antibodies directed against double-stranded DNA (anti-dsDNA) and Smith antigen (anti-Sm).

The core immunologic injury in SLE is Type III hypersensitivity, where antigen-antibody immune complexes form in the circulation and deposit in tissues such as the kidneys, skin, joints, and serous membranes. Once lodged in vessel walls and basement membranes, these immune complexes activate the classical complement pathway (C1q, C4, C2, then C3), recruiting inflammatory cells and causing tissue destruction. This is why complement levels (C3 and C4) drop during active disease flares, a key testable concept. Rising anti-dsDNA titers with falling complement levels together signal impending or active flare.

The predilection for the kidneys is particularly important. Immune complexes deposit in the glomerular basement membrane, producing a spectrum of glomerulonephritis known as lupus nephritis. The classic histologic finding is the "wire-loop" thickening of capillary walls seen in diffuse proliferative glomerulonephritis (Class IV), which is the most common and most severe renal lesion in SLE.

The malar rash occurs because ultraviolet light induces apoptosis of keratinocytes, releasing nuclear antigens and triggering local immune complex deposition and inflammation. This explains why sun exposure is both a trigger and an exacerbating factor. The rash characteristically spares the nasolabial folds, distinguishing it from rosacea, which involves those folds.

Joint involvement in SLE is a non-erosive, non-deforming arthritis affecting small joints, which contrasts with rheumatoid arthritis (RA). When deformities do occur (Jaccoud arthropathy), they are reducible because the damage is to periarticular structures rather than to cartilage and bone.

Hematologic manifestations are caused by autoantibodies directed against blood cell lines: anti-red blood cell antibodies cause autoimmune hemolytic anemia (Coombs-positive), anti-platelet antibodies cause immune thrombocytopenia, and anti-lymphocyte antibodies cause lymphopenia. The presence of antiphospholipid antibodies (anticardiolipin, lupus anticoagulant, anti-beta-2 glycoprotein I) explains the association with recurrent thrombosis and pregnancy loss, and also produces a false-positive VDRL or RPR for syphilis.

Serositis (pleuritis and pericarditis) results from immune complex deposition on serosal surfaces, causing chest pain that is often pleuritic in character.

02Classification and Clinical Manifestation

2019 EULAR/ACR Classification Criteria

Classification requires a positive ANA at a titer of 1:80 or greater as an entry criterion. Once met, clinical and immunologic domains are scored. A total score of 10 or more classifies SLE. Only the highest-weighted criterion within each domain is counted.

Clinical Manifestation by Organ System

WHO Classification of Lupus Nephritis

03Diagnostic Workup

The diagnostic workup for SLE follows a layered approach. When you encounter a young woman with multisystem complaints (joint pain, rash, fatigue, renal abnormalities), the best initial test is ANA. ANA is extremely sensitive (greater than 95%) but not at all confirmatory, because it is positive in many conditions including other autoimmune diseases, infections, medications, and even in healthy individuals. A negative ANA, however, carries strong negative predictive value and effectively rules out SLE in most clinical scenarios.

Once ANA is positive, the next step is to order confirmatory antibodies. Anti-dsDNA antibodies are present in approximately 70% of patients with SLE and are particularly useful because their titers correlate with disease activity, especially with lupus nephritis. Rising anti-dsDNA titers with simultaneously falling C3 and C4 levels is a classic pattern signaling an active flare. Anti-Smith antibodies are the most confirmatory serologic marker for SLE. Although they are found in only about 25 to 30% of patients, a positive anti-Smith result is nearly pathognomonic.

Complement levels (C3 and C4) should be checked at baseline and monitored longitudinally. In SLE, complement is consumed through the classical pathway during immune complex formation. Low C3 and C4 during a flare help distinguish active lupus from other causes of symptom worsening (such as infection, where complement may be normal or elevated as an acute-phase reactant).

A complete blood count will reveal cytopenias. Autoimmune hemolytic anemia should be confirmed with a direct Coombs test and peripheral smear showing spherocytes and reticulocytosis. Thrombocytopenia and lymphopenia are common and contribute to the classification score.

Urinalysis is essential at diagnosis and during follow-up. The presence of proteinuria (especially greater than 0.5 g per 24 hours), hematuria, or RBC casts raises concern for lupus nephritis and warrants further evaluation. The gold standard for diagnosing and classifying lupus nephritis is renal biopsy, which determines the WHO/ISN class and directly guides treatment decisions. A biopsy should be performed when there is clinical or laboratory evidence of renal involvement.

Antiphospholipid antibodies (lupus anticoagulant, anticardiolipin IgG/IgM, anti-beta-2 glycoprotein I) should be tested because of the strong association between SLE and antiphospholipid syndrome. These antibodies cause a false-positive VDRL or RPR (a frequently tested fact) and are associated with recurrent arterial and venous thrombosis as well as pregnancy loss.

One notable lab pattern to remember: ESR is often elevated in SLE, but CRP may remain normal unless there is concurrent serositis or infection. A significantly elevated CRP in a patient with known SLE should prompt evaluation for an infectious cause rather than attributing it solely to a lupus flare.

04Management and Treatment

Hydroxychloroquine (HCQ) is the cornerstone of SLE therapy and should be prescribed to every patient with SLE unless there is a true contraindication. At a dose of 200 to 400 mg daily (not exceeding 5 mg/kg actual body weight per day to minimize retinal toxicity), HCQ reduces the frequency and severity of flares, decreases the risk of organ damage accumulation, improves survival, has antithrombotic properties, and has a favorable lipid effect. Patients on HCQ require annual ophthalmologic screening (typically starting after 5 years of use or sooner if there are additional risk factors) to monitor for hydroxychloroquine retinopathy, which can cause irreversible visual loss.

For mild disease such as arthralgias, mild arthritis, skin manifestations, and fatigue, NSAIDs can be used for symptom relief, and low-dose prednisone (less than or equal to 7.5 mg/day) may be added for short periods. Topical corticosteroids or calcineurin inhibitors can be applied to cutaneous lesions. The overarching principle is to minimize corticosteroid exposure because chronic steroid use leads to avascular necrosis, osteoporosis, diabetes, cataracts, and infections, all of which contribute substantially to long-term morbidity in SLE.

For moderate to severe disease (significant serositis, cytopenias, neuropsychiatric involvement), higher-dose corticosteroids are used (prednisone 0.5 to 1 mg/kg/day), with rapid tapering once disease control is achieved. A steroid-sparing immunosuppressant such as azathioprine (1 to 3 mg/kg/day), methotrexate (7.5 to 25 mg weekly), or mycophenolate mofetil is introduced to allow steroid reduction.

Lupus nephritis (Class III or IV) demands aggressive treatment. Induction therapy consists of pulse IV methylprednisolone (500 to 1000 mg/day for 3 days), followed by oral prednisone starting at 0.5 to 1 mg/kg/day with a gradual taper. The immunosuppressive backbone is either mycophenolate mofetil (MMF) at 2 to 3 g/day or IV cyclophosphamide (the Euro-Lupus low-dose protocol: 500 mg IV every 2 weeks for 6 doses, or the NIH high-dose protocol). MMF is now generally preferred for induction in most patients because of comparable efficacy with a more favorable side-effect profile, particularly regarding gonadal toxicity and infection risk. Voclosporin (23.7 mg twice daily) is a newer calcineurin inhibitor approved as an add-on to MMF and corticosteroids for lupus nephritis, shown to improve renal response rates.

Maintenance therapy after successful induction uses MMF (1 to 2 g/day) or azathioprine (2 mg/kg/day) for a minimum of 3 years, and often longer if disease remains active. HCQ is continued throughout.

Belimumab, a monoclonal antibody targeting B-lymphocyte stimulator (BLyS/BAFF), is approved for active SLE not adequately controlled by standard therapy and for active lupus nephritis as add-on treatment. Rituximab (anti-CD20) is used off-label for severe, refractory cases such as refractory cytopenias or nephritis that does not respond to standard regimens, though randomized trial data for lupus nephritis have been mixed.

Pregnancy considerations are heavily tested. Hydroxychloroquine must be continued during pregnancy because discontinuation increases flare risk. Azathioprine is considered safe during pregnancy and lactation. Mycophenolate mofetil, cyclophosphamide, and methotrexate are absolutely contraindicated in pregnancy due to teratogenicity. Patients planning conception should switch from MMF to azathioprine well before attempting pregnancy. Women with antiphospholipid antibodies require low-dose aspirin and prophylactic heparin during pregnancy to reduce the risk of pregnancy loss and pre-eclampsia. Anti-Ro (SSA) and anti-La (SSB) antibodies in the mother carry a risk of neonatal lupus, which can cause congenital heart block in the fetus, an important association tested on exams.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most common trap with SLE questions is confusing drug-induced lupus with true SLE. When a vignette describes a patient on hydralazine or procainamide who develops arthritis, serositis, and a positive ANA, students often jump to SLE. The distinguishing clue is the antibody profile: drug-induced lupus is associated with anti-histone antibodies, while anti-dsDNA and anti-Smith are absent. Furthermore, drug-induced lupus almost never causes renal or CNS disease, and symptoms resolve when the offending drug is stopped.

Another frequently tested pitfall is the false-positive syphilis test. When a vignette includes a young woman with joint pain, rash, and a positive RPR, the test-writer is checking whether you recognize that antiphospholipid antibodies in SLE cause a false-positive nontreponemal test. The confirmatory treponemal test (FTA-ABS) will be negative. Students who do not recall this association will incorrectly diagnose syphilis.

A third trap involves distinguishing between a lupus flare and infection in a patient on immunosuppressive therapy. Both can present with fever and elevated inflammatory markers. The key is that active lupus flares typically show a rising anti-dsDNA, falling complements, and an ESR that is elevated out of proportion to CRP. An infection in an immunosuppressed lupus patient tends to show a significantly elevated CRP, positive cultures, and stable or improving complement levels.

Students also stumble on the renal biopsy question. When a vignette describes a patient with known SLE who develops new-onset proteinuria, hematuria, and rising creatinine, the next best step is renal biopsy, not empiric immunosuppression. The biopsy is essential because the WHO class directly determines treatment intensity, and you cannot reliably distinguish Class III from Class IV from Class V on clinical and laboratory grounds alone.

Finally, watch for questions about pregnancy in SLE. The tested concept is knowing which medications are safe and which are teratogenic. Hydroxychloroquine is continued. Azathioprine is safe. Mycophenolate mofetil, cyclophosphamide, and methotrexate must be stopped. Anti-Ro/SSA antibodies in the mother create a risk of neonatal lupus with congenital heart block. This last point often appears as a standalone question asking what complication to monitor for in a pregnant patient with SLE and positive anti-Ro antibodies.

The core competency being tested across SLE questions is your ability to integrate multisystem findings into a single unifying diagnosis, correctly sequence the diagnostic workup from screening (ANA) to confirmation (anti-dsDNA, anti-Smith, renal biopsy), and apply the right treatment intensity based on organ involvement and disease severity.