Scaling plaques in a patient with chronic kidney disease
A 58-year-old man with stage 4 chronic kidney disease (eGFR 24 mL/min/1.73 m²) on amlodipine, atorvastatin, calcium carbonate, and erythropoietin presents with a 3-month history of well-demarcated, erythematous, silvery-scaled plaques over both elbows, knees, and the sacrum. Body surface area is approximately 8%. He has no joint pain, no nail pitting, and no personal or family history of psoriasis. He reports mild pruritus. A potassium hydroxide preparation from a knee plaque is negative. He was started on topical clobetasol propionate 0.05% ointment twice daily two weeks ago with minimal improvement, and his primary care physician asks whether methotrexate can be started.
1. What is your differential diagnosis and what bedside or laboratory steps would you take before committing to a diagnosis of plaque psoriasis?
Read the worked answer
The morphology (well-demarcated erythematous plaques with silvery scale on extensor surfaces and sacrum) strongly suggests psoriasis, but several features demand caution: absence of nail changes and family history, modest response to a superpotent topical steroid, and a comorbidity profile that generates psoriasis mimics.
Differential
- Psoriasis vulgaris - still the leading diagnosis.
- Lichen simplex chronicus / prurigo nodularis - chronic scratching can produce lichenified plaques; look for exaggerated skin markings and excoriations.
- Tinea corporis - KOH was negative, but a single negative scrape has limited sensitivity; repeat with scrapings from the active edge, and consider a periodic acid–Schiff stain if a biopsy is done.
- Acquired perforating dermatosis (Kyrle disease) - strongly associated with advanced CKD and dialysis; lesions are often pruritic follicular/parafollicular keratotic papules, sometimes coalescing into plaques. Examine for umbilicated keratotic plugs and consider biopsy.
- Nummular eczema - usually more exudative, less sharply demarcated, and intensely pruritic.
- Drug eruption - atorvastatin and amlodipine are uncommon culprits but temporally plausible; a morbilliform pattern would be expected rather than plaques.
- Secondary syphilis - consider serology given the broad differential of scaling plaques.
Steps before committing
- Full skin and nail exam including scalp, gluteal cleft, and intertriginous areas; document PASI and DLQI.
- Repeat KOH from the active edge of a plaque, and consider fungal culture.
- Skin biopsy (punch or shave) of an established plaque for histology if diagnosis is uncertain - psoriasis shows regular acanthosis, hypogranulosis, parakeratosis with neutrophils, and dilated capillary loops.
- Baseline bloods: CBC, renal function, liver function, hepatitis B and C serology, HIV, fasting lipids, HbA1c, and TB screening (IGRA) in anticipation of systemic therapy.
- Assess for psoriatic arthritis (joint exam, history of dactylitis/enthesitis) and cardiovascular risk factors.
A therapeutic trial of topical therapy is reasonable, but the CKD context should raise the possibility of perforating dermatosis if the morphology is not classic.
2. Assuming the diagnosis is confirmed as moderate plaque psoriasis, how does his CKD affect your choice of systemic agent? Address methotrexate specifically.
Read the worked answer
Methotrexate is a poor choice here. It is renally cleared, and in eGFR <30 mL/min/1.73 m² accumulation leads to markedly increased risk of myelosuppression, mucositis, hepatotoxicity, and pneumonitis. Even at reduced doses, the margin of safety is unacceptable without nephrology input and close monitoring; most guidelines advise avoiding it in severe renal impairment.
Agent-by-agent reasoning
- Phototherapy (NB-UVB): Preferred first-line systemic-equivalent option. No renal clearance issue; requires access and time commitment. Monitor for erythema and cumulative carcinogenic risk.
- Acitretin: Hepatically metabolised, no renal dose adjustment needed. Useful in CKD. Caveats: teratogenicity (not relevant here), mucocutaneous dryness, hyperlipidaemia (already on a statin - monitor lipids), and hepatotoxicity. Dose 10–25 mg daily.
- Apremilast: Renally excreted; dose must be halved to 30 mg once daily when CrCl <30 mL/min/1.73 m². Reasonable option with GI and mood side effects to counsel on.
- Biologics: TNF inhibitors (adalimumab, etanercept) and IL-17/IL-23 inhibitors are large proteins not renally cleared and are generally usable in CKD. Screen for TB and hepatitis first. IL-17 inhibitors may be preferred if psoriatic arthritis emerges.
- Cyclosporine: Avoid - nephrotoxic and hypertensive, unacceptable in stage 4 CKD.
- Hydroxyurea: Renally excreted; avoid or use with extreme caution.
Practical plan
- Optimise topicals (clobetasol with occlusion, calcipotriol combination) and consider NB-UVB.
- If systemic needed, favour acitretin or a biologic over methotrexate or cyclosporine.
- Coordinate with nephrology regarding drug dosing, and review amlodipine/atorvastatin only if a drug eruption becomes plausible.
- Document a clear rationale in the notes, since the referring physician's request for methotrexate must be declined with explanation.