Calcium phosphate deposits
Calcium phosphate deposits within the kidney may be seen in the setting of hypercalcemia/ hypercalciuria or hyperphosphatemia/ hyperphosphaturia. Regardless of the etiology, the result is the formation of hydroxyapatite crystals within the tubular lumens (Fig 1 and 2), tubular epithelium, tubular basement membranes or within the interstitium. The term nephrocalcinosis is usually reserved to describe the presence of tubulointerstitial calcium phosphate deposits in patients with hypercalcemic disorders. These include, among others, hyperparathyroidism, sarcoidosis, underlying malignancies of various types, milk-alkali syndrome, hypervitaminosis A or D, Dent disease and other tubulopathies.
On the other hand, phosphate nephropathy is predominantly seen in patients taking oral sodium phosphate as a purgative for bowel preparation, but may also be seen in other conditions such as nephrotic syndrome and phosphate supplementation. While there is morphologic overlap among these two entities, it may be possible to differentiate them by light microscopy (see reference). Calcium-type deposits are reported to have a finely granular or clumpy morphology, while phosphate-type deposits tend to have shell-like or globular deposits. Clinicopathological correlation nevertheless is required to determine the etiology in these patients.
Wiech T, Hopfer H, Gaspert A, et al. Histopathological patterns of nephrocalcinosis: a phosphate type can be distinguished from a calcium type. Nephrol Dial Transplant. 2012; 27: 1122-1131
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