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:

Tubule containing an intraluminal crystalline cast (H&E). Figure 1A from Makkapati et al, AJKD © National Kidney Foundation.

Viewing the H&E stain under polarized light reveals that these crystals appear refractile:

Polarization at low power shows abundant refractile oxalate casts within tubular lumina (H&E). Figure 1C from Makkapati et al, AJKD © National Kidney Foundation.

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.

High-Oxalate–Containing Foods. Table 1 from Makkapati et al, AJKD © National Kidney Foundation.

– Post prepared by Timothy Yau, AJKD Social Media Editor. Follow him @Maximal_Change.

To view the Makkapati et al Teaching Case (subscription required), please visit AJKD.org.

Title: “Green Smoothie Cleanse” Causing Acute Oxalate Nephropathy
Authors: S. Makkapati, V.D. D’Agati, and L. Balsam
DOI: 10.1053/j.ajkd.2017.08.002