A recent AJKD Teaching Case by Makkapati et al describes a 65-year-old patient with Roux-en-Y gastric bypass who developed acute kidney injury (AKI) in the setting of a 10-day “green smoothie cleanse.“
Histology from her kidney biopsy demonstrate numerous calcium-oxalate crystals (gray-white, sheaf-shaped) within the tubular lumina:
Viewing the H&E stain under polarized light reveals that these crystals appear refractile:
This property of oxalate crystals to refract light when polarized is called birefringence. Although the gray-white appearance on H&E and birefringence is usually sufficient to distinguish calcium oxalate from calcium phosphate, another technique is the von Kossa stain. In the setting of acute phosphate nephropathy (as has been classically described in patients with CKD receiving phosphate-based bowel preparations), the von Kossa will stain the phosphate salts a brown/black color. In calcium oxalate nephropathy, the crystals will be negative on von Kossa.
Patients who are at risk for AKI from acute hyperoxaluria include those with increased absorption of oxalate from fat malabsorption syndromes or Roux-en-Y gastric bypass. Additionally, use of antibiotics can lead to a deficiency of oxalate degrading bacteria (oxalobacter formigenes) in the gut. Lastly, ingestion of oxalate rich foods leads to increased intestinal absorption of oxalate.
Title: “Green Smoothie Cleanse” Causing Acute Oxalate Nephropathy
Authors: S. Makkapati, V.D. D’Agati, and L. Balsam