Fanconi Syndrome Workup

Updated: Mar 16, 2023
  • Author: Sahar Fathallah-Shaykh, MD; Chief Editor: Craig B Langman, MD  more...
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Laboratory Studies

The diagnosis of Fanconi syndrome is made based on tests that document the excessive loss of substances in the urine (eg, amino acids, glucose, phosphate, bicarbonate) in the absence of high plasma concentrations. More elaborate tests are designed to determine the renal threshold for these substances (ie, the concentration in the blood at which these substances appear in the urine) or their fractional reabsorption (ie, the percentage of the filtered load that is reabsorbed by the renal tubule). The results are compared with normal levels for age. Recently, an increased urinary lactate-to-creatinine ratio has been postulated as a sensitive test for disordered proximal tubular function. Moreover, urinary retinol-binding protein 4 is thought to be a functional biomarker of the proximal tubule and elevated levels appears to be an excellent screening test for Fanconi syndrome. [24]  

The choice of tests designed to determine the cause of the syndrome should be based on information obtained by means of the history or physical examination. When toxic agents such as drugs or heavy metals are suspected, measure their levels in the blood or urine. When cystinosis appears to be the cause, measure the cellular concentrations of cystine in fibroblasts or leucocytes. Enzymatic defects can be ascertained by measuring tissue levels. An international consensus document was published in 2014 that provides expert opinion regarding diagnosis and treatment of nephropathic cystinosis. [25]  


Imaging Studies

Imaging studies do not play an important role in the diagnosis or follow-up of patients with Fanconi syndrome. Rarely, a patient may come to the attention of the physician because of rickets, a consequence of mineral losses in the urine and disturbances in vitamin D metabolism.


Histologic Findings

Most patients with Fanconi syndrome have no distinguishing features of renal morphology.

A distortion in the appearance of the proximal tubule, which resembles a swan neck, was described in children with cystinosis and was considered to be the cause of the syndrome. However, this distortion is more likely to be consequence of cellular atrophy secondary to the deposition of cystine.

Nonspecific glomerular and tubular changes are also described in Lowe syndrome.