Diagnostic Utility of Urinary Sediments in Light-Chain Proximal Tubulopathy (LCPT): Report of Three Cases

Introduction: LCPT is characterized by crystals within proximal tubular epithelial cells and is typically associated with Fanconi syndrome. However, in cases without Fanconi syndrome, diagnosing LCPT is challenging. Microscopic analysis of urinary sediments may be valuable for LCPT, as cells and casts might contain light chain-derived crystals. Although their diagnostic value remains underreported, we present three cases of LCPT without Fanconi syndrome, where urinary sediments with crystals were crucial in understanding their condition. Case Description: A 54-year-old female with macroglossia, carpal tunnel syndrome, subcutaneous nodules, and IgA-κ type M protein was diagnosed with AL amyloidosis. Urine protein was 5.3 g/gCr, and serum creatinine (sCr) was 1.09 mg/dL. However, urinary sediments revealed crystals within tubular epithelial cells, inconsistent with amyloidosis. Kidney biopsy was negative for amyloid deposition, and LCPT was diagnosed. A 55-year-old male with psoriasis and hypertension experienced a gradual decline in renal function. sCr level was 1.92 mg/dL. Urinary sediments revealed crystals within tubular epithelial cells and casts. Light chain staining of the sediments was positive for kappa light chain only. IgG-κ type M protein was identified, and kidney biopsy demonstrated LCPT with significant tubular casts containing crystals. A 66-year-old female presented with fever, edema, and impaired renal function. sCr level was 2.41 mg/dL. IgG-κ type M protein was detected. Urinary sediments were suspicious for crystals within tubular epithelial cells and macrophages. Immunostaining and electron microscopic analysis of cell blocks confirmed crystals in both cell types. Kidney biopsy disclosed LCPT and crystals within interstitial macrophages. Moreover, a biopsy of a subcutaneous soft mass detected histiocytes containing crystals. These findings led to a diagnosis of Crystal-Storing Histiocytosis. Discussion: Direct observation of tubular epithelial cells and casts in urine supported the diagnosis of LCPT. Immunostaining and electron microscopic analysis were informative for identifying the origin of crystals, the type of light chain, and the cell type containing crystals, deepening the understanding of the patient’s condition without invasive kidney biopsy. Further consideration is required.