A 79-year-old Caucasian man presented with acute renal failure. A review of systems was positive foranorexia, nausea, and vomiting, but was otherwise negative. His blood pressure was 156/88 mm Hg, andurine output over 24 hours was 1.7 L. His medical history was remarkable for hypertension and possibleupper respiratory infection about 2 weeks prior to admission, with cutaneous rash after antibiotic use. Hisserum creatinine level was 10 mg/dL (884 �mol/L); blood urea nitrogen, 57 mg/dL (20.3 mmol/L); sodium,135 mEq/L (mmol/L); potassium, 4.6 mEq/L (mmol/L); chloride, 95 mEq/L (mmol/L); bicarbonate, 27mEq/L (mmol/L); calcium, 9.2 mg/dL (2.30 mmol/L); phosphorus, 5.9 mg/dL (1.91 mmol/L); and albumin,3.2 g/dL (32 g/L). He was anemic with a hematocrit value of 27.9% and hemoglobin level of 9.7 g/dL (97g/L); white blood cell and platelet counts were 12,200 � 103/�L (�109/L) and 157,000 � 103/�L (�109/L),respectively. Antinuclear antibodies were positive at 1:40, antineutrophil cytoplasmic antibodies werenegative, and C3 was 127 mg/dL. Urinalysis showed specific gravity of 1.006, pH of 7.5, no protein, and 4white blood cells and 1 red blood cell per high-power field.

What is your clinical differential diagnosis?

The clinical differential diagnosis included drug-induced acute interstitial nephritis.

Figures 29A and 29B. What do you see by light microscopy? (Glomeruli [not shown] were unremarkable.)

There is a marked interstitial infiltrate containing numerous eosinophils with focal granulomatous reaction (Fig29A; Jones’ silver stain, original magnification �200). Some tubular profiles contained eosinophils within thetubular epithelium (Fig 29B; Jones’ silver stain, original magnification �1,000).

Figures 29C and 29D. What additional features do you observe by light microscopy?

Other tubules contain casts with sharply delineated borders with surrounding syncytial giant cell formation (Fig29C; Jones’ silver stain, original magnification �400). Silver positive spicules protrude from the border of one ofthese casts with surrounding inflammatory cell reaction (Fig 29D; Jones’ silver stain, original magnification�1,000). Interestingly, Congo red stain was positive in this area, but not elsewhere.

AJKD QUIZ PAGE ANSWERSJULY 2004

What is your diagnosis, and what additional laboratory test(s)do you think would be useful in this setting?

These light microscopic and immunofluorescence findings are diagnostic of light chain castnephropathy. Although no amyloid deposits were detected in glomeruli, vessels, or interstitium, thepatient may develop amyloid in the future because the monoclonal � light chain in this patient mayalso have amyloidogenic properties as shown by the amyloid staining present in the cast. Lightmicroscopy also showed numerous eosinophils in the interstitium with granulomatous reaction andeosinophils detected also within the tubular epithelium. Although eosinophils can be seen in lightchain cast nephropathy as part of the inflammatory response, in this patient with history ofcutaneous rash after antibiotic administration, the eosinophils and the granulomatous reactionmay represent morphologic findings of a superimposed drug-induced hypersensitivity reaction.

Additional useful tests would include serum protein electrophoresis and urine protein electro-phoresis (with immunofixation if needed to detect the underlying monoclonal protein) and bonemarrow biopsy. In this patient, a bone marrow biopsy was within normal limits, but serum andurine immunofixation revealed monoclonal � light chain.

Final Diagnosis: Light chain cast nephropathy.

Case provided by Agnes B. Fogo, MD,Michele Rossini, MD, Vanderbilt UniversityMedical Center, Nashville, TN; and AndrewLazin, MD, Gulf Coast Kidney Associates,Venice, FL.

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Figure 29E. What do you see by immunofluorescence microscopy?

By immunofluorescence, there is 3� staining for � light chain in the casts with only trace staining for �. There isno staining in the glomerular or tubular basement membranes (Fig 29E; anti-� antibody, original magnification�200). Immunoglobulin G (IgG), IgA, IgM, C3, and C1q staining were negative.

AJKD QUIZ PAGE ANSWERS(continued)