Phosphate nephropathy or nephrocalcinosis is an adverse renal condition that arises with a formation of phosphate crystals within the kidney's tubules. This renal insufficiency is associated with the use of oral sodium phosphate (OSP) such as C.B. Fleet's Phospho soda and Salix's Visocol, for bowel cleansing prior to a colonoscopy. According to the U.S. Food and Drug Administration (FDA), the potential risk factors of this complication include pre-existing kidney disease, increased age, female gender, dehydration, comorbidities such as diabetes mellitus and hypertension, and concurrent treatment with hypertensive medications (ACE inhibitors and angiotensin receptor blockers) and medications that affect renal perfusion (Nonsteroidal anti-inflammatory drug or NSAIDs and diuretics). This complication can be diagnosed with renal tests and biomarkers in laboratories including histochemical staining of renal biopsy specimens, the measure of creatinine level, GFR level, and urine output, urine microscopy, CT scanning, and urinalysis. Phosphate nephropathy may also lead to further renal complications including acute kidney diseases and chronic kidney diseases, or the abrupt and gradual loss of kidney function over time. Different management approaches involve the use of alternative bowel preparation agents and increasing patients' risk assessment among healthcare professionals, including nephrologists, gastroenterologists, and renal pathologists. Other agents used for bowel preparation (e.g. magnesium citrate or PEG-3350 & electrolyte-based purgatives such as Colyte or Golytely) do not carry this risk. According to the U.S. Food and Drug Administration (FDA), "acute phosphate nephropathy is a rare, serious adverse event that has been associated with the use of OSPs. The occurrence of these events was previously described in an Information for Healthcare Professionals sheet and an FDA Science Paper issued in May 2006. Additional cases of acute phosphate nephropathy have been reported to FDA and described in the literature since these were issued."
Signs and symptoms Patients with phosphate nephropathy have variable outcomes, either being recognized as developing acute kidney injury, the sudden onset of renal failure, or being undiagnosed. As the deposition of calcium phosphate crystals are detected at the renal tubules following the use of OSP, the symptoms of phosphate nephropathy are similar to acute tubular necrosis, an intrinsic renal injury. For example, events including diarrhea, vomiting, dehydration, sepsis, and hypotension following the colonoscopy, can indicate the risk of phosphate nephropathy and raise the concern for acute tubular necrosis. The outcomes of hypotension and dehydration are dry mucous membrane, decreased skin turgor, and cold extremities, which can be used to notify the abnormal renal perfusion. As there is a gap between the first administration of OSP and the onset of recognizable symptoms, many cases of phosphate nephropathy are overlooked and not recommended to biopsy for further investigation. The transient hyperphosphatasemia, the electrolyte disorder with elevated phosphate level in the blood, is found to be correlated with the use of OSP after colonoscopy. Significant electrolyte abnormalities including hypocalcemia, hypernatremia, and hypomagnesemia are also the outcomes of the use of OSP. As these detection tests are mostly operated at the laboratory level, phosphate nephropathy incidents are widely under-recognized and overlooked.
Risk factors According to the FDA, caution is required to consider prior use of OSP, especially in patients with the following potential risk factors. For instance, people older than 55, female gender, people with a history of kidney diseases if their GFR level is less than 60 mL/min, people who recurrently on antihypertensive treatment with NSAIDs, ARBs, ACEIs, and diuretics, people who have lower fluid intake and less bowel movements, people who have underlying systemic and gastrointestinal diseases, and a short interval between OSP administrations (less than 12 hours interval). The risk of acquiring phosphate nephropathy was reported to increase in parallel with the number of these listed risk factors. Older people are particularly at risk when using OSP for colonoscopy, as they have lower fluid intake, have intrinsically less bowel movements, and often have antihypertensive or nephrotoxic drugs. It is also reported that people with comorbidities including diabetes mellitus, hypertension, and other metabolic syndromes will have a higher risk of phosphate nephropathy.
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