Proteinuria, low C3, and a positive ASO: lupus nephritis or post-streptococcal glomerulonephritis?
A 15-year-old girl presents with 3 months of fatigue, a malar rash, oral ulcers, and arthralgia, plus 2 weeks of leg swelling. Blood pressure is 138/88 mmHg. Urinalysis shows 3+ protein, 20–30 RBC/hpf with casts. Creatinine is 0.9 mg/dL, albumin 2.8 g/dL, C3 40 mg/dL (low), C4 12 mg/dL (low), ANA 1:1280, anti-dsDNA 300 IU/mL, platelets 110,000/µL, and WBC 3.2 × 10³/µL. She had a sore throat 3 weeks ago and ASO is positive. She is sexually active and asks about birth control.
1. How do you distinguish lupus nephritis from post-streptococcal glomerulonephritis here, and what is your next step?
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The malar rash, oral ulcers, cytopenias, high-titer ANA, and anti-dsDNA make SLE the unifying diagnosis. Post-streptococcal glomerulonephritis typically produces low C3 with a normal C4, negative ANA and anti-dsDNA, and resolves over 6–8 weeks; this girl has low C3 and C4, strongly suggesting classical complement pathway consumption by immune complexes. A positive ASO reflects recent streptococcal exposure, not causation. Thrombocytopenia and leukopenia also argue against PSGN.
Next step is kidney biopsy before immunosuppression, to classify proliferative (class III/IV) versus membranous disease and guide induction. Start hydroxychloroquine, an ACE inhibitor, and blood pressure control now. Do not treat empirically with high-dose cyclophosphamide before histology unless she is rapidly deteriorating.
2. Biopsy shows class IV lupus nephritis. Outline induction and the safety counseling this teenager specifically needs.
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For class IV nephritis, induce with IV methylprednisolone followed by oral prednisone, plus mycophenolate mofetil or low-dose IV cyclophosphamide. Add hydroxychloroquine for all patients (≤5 mg/kg/day actual weight, with baseline and annual ophthalmologic screening). Start an ACE inhibitor or ARB for proteinuria and control blood pressure.
Counseling: mycophenolate is teratogenic-check pregnancy before starting, require reliable contraception (two methods or a long-acting method), and discuss drug interactions with combined oral pills. Avoid NSAIDs with renal involvement, and emphasize sun protection because UV exposure triggers flares. Live vaccines are contraindicated on immunosuppression; give inactivated influenza and appropriate pneumococcal vaccines. Explain that hydroxychloroquine prevents flares and must not be stopped.
3. At 6 months she feels well, proteinuria is trace, and she wants to stop prednisone and hydroxychloroquine. How do you handle this?
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Feeling well is not proof of remission. Confirm with complement levels, anti-dsDNA, urine protein-to-creatinine ratio, and blood pressure. If these are normal, taper prednisone slowly to the lowest dose or off, but never stop hydroxychloroquine: it reduces flares, renal relapse, and thrombosis, and stopping it raises relapse risk. Emphasize that nephritis can smolder silently; routine urine and complement monitoring is non-negotiable.
Address adherence honestly-teenagers often stop steroids because of weight gain and acne. Acknowledge this, offer dietician and dermatology support, and reinforce contraception if she remains on mycophenolate. Plan transition to adult rheumatology and nephrology before age 18 so she does not fall between services.