At a Glance
The clinical findings of (renal) Fanconi syndrome are that of polyuria, polydipsia, failure to thrive and risk of dehydration in infancy and/or early child hood. At the age of weight bearing, it is not uncommon to see a child with “bowed legs” as they begin to walk.
What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?
Laboratory findings of a plasma nongap metabolic acidosis, hypokalemia, and hypophosphatemia (often missed in the United States because the “panel” of routine blood work does not contain a phosphorus). Analysis of the urine reveals a low specific gravity (despite evidence of dehydration), amino aciduria (seen as a “positive” dip stick for protein), glycosuria and, if measured, phosphaturia. Causes of (renal) Fanconi syndrome are rare, but the most common is cystinosis, although other causes do occur.
Test Results Indicative of the Disorder
The findings of a low CO2 (metabolic acidosis), low phosphorous (with evidence of a high urinary phosphorous) and a low potassium (with evidence of a high urinary potassium) should alert the clinician of the possibility of Fanconi syndrome.
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What Lab Results Are Absolutely Confirmatory?
The clinical spectrum of inability to concentrate the urine, amino aciduria, glycosuria, phosphatura with additional blood findings of metabolic acidosis, hypokalemia and hypophosphatemia guide the work up. There are many causes of (renal) Fanconi syndrome, including being medication induced (e.g. amphotericin use) and genetic disease (e.g. cystinosis, tyrosemia, Lowes syndrome, Zellwegers syndrome). An ophthalmologic evaluation may show evidence of retinal crystals that may represent cysteine crystals. A white blood cell cysteine level is confirmation for cystinosis. Other causes of (renal) Fanconi syndrome require specific workup by a metabolic physician or a hepatologists.
What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?
Long-term care is based on the underlying cause. These children may require 2-4 times more fluid than normal with aggressive bicarbonate, potassium and phosphorous replacement. It is important to identify the underlying cause that also will need treatment. Some children may require future renal or hepatic transplantation, depending on the etiology of the Fanconi syndrome.
Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications – OTC drugs or Herbals – that might affect the lab results?
Expired tetracycline, amphotericin and some chemotherapeutic agents may cause tubular damage resulting in a (renal) Fanconi-like syndrome.
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