Gaddy A., Schwantes-An T.-H., Moorthi R.N., Phillips C.L., Eadon M.T., Moe S.M.:Incidence and Importance of Calcium Deposition in Kidney Biopsy Specimens. American Journal of Nephrology DOI 10.1159/000525647
Calcium deposition in the kidney parenchyma (using special stains such as the von Kossa stain), also known as dystrophic calcification, is a relatively common finding (about 10%) in native kidney biopsy specimens. The deposits are typically calcium oxalate (CaOx; birefringent on polarized light) or calcium phosphate (CaP; non-birefringent on polarized light) or mixed CaOx/CaP. The prognostic significance of these deposits is not well understood. The von Kossa stain does not directly detect calcium but rather a phosphate complexed with calcium. Pure CaOx does not stain positively with the von Kossa stain (unless the tissue is pre-treated by an oxidant such as H2O2), but mixed CaOx/CaP will be positive [see Pizzalato P, 1964]. Alizarin S staining is required to directly confirm the presence of calcium in deposits.
Gaddy and colleagues sought to better understand the relationship of kidney parenchyma calcium deposition to co-morbidity and prognosis in a retrospective study of 385 patients with calcium deposition in native kidney biopsies (CaOx = 125; CaP = 230; and mixed CaOx/CaP = 30). About 9.8% of all native kidney biopsies showed some evidence of calcium deposition. As expected, CaOx deposits were found more commonly in gastric bypass, malabsorption, or vitamin D intake. Patients with any calcification were older, had more comorbidities and had lower eGFR and a background of prior acute tubular necrosis and acute kidney injury. Importantly, the subjects with any calcification had a slower rate of decline in eGFR perhaps due to the high prevalence of AKI and recovery. This is different than the adverse prognostic implications of calcium deposition in transplant kidney biopsies. It was not possible in this study to examine the differential prognostic implications of CaOx versus CaP deposition. These are intriguing findings. The extreme heterogeneity of dystrophic calcification probably precludes precise determination of its prognostic implication.