Interferences of homogentisic acid (HGA) on routine clinical chemistry assays in serum and urine and the implications for biochemical monitoring of patients with alkaptonuria.

OBJECTIVES: We have assessed the effect of elevated concentrations of homogentisic acid (HGA) as in alkaptonuria (AKU), on a range of routine chemistry tests in serum and urine. DESIGN AND METHODS: HGA was added to pooled serum and a range of assays was analysed with Roche Modular chemistries. Effects on urine were assessed by diluting normal urine with urine from a patient with AKU, adding HGA to urine and after lowering output of urinary HGA with nitisinone treatment.
RESULTS: Serum enzymatic creatinine showed 30% negative interference with 100μmol/L HGA and >50% at 400μmol/L. Serum urate 100 to 480μmol/L was reduced up to 20% at 100 and to 50% with 400μmol/L HGA. Serum cholesterol between 3 and 11mmol/L was reduced by 0.5mmol/L with 400μmol/L HGA. Urine enzymatic creatinine and urate with >2mmol/L HGA showed concentration dependent negative interference up to 80%. A positive interference in urine total protein by benzethonium turbidometric assay was observed, with 10mmol/L HGA equivalent to 1g/L protein. Jaffe creatinine, Na, K, Cl, Mg, Ca, phosphate, ALT, GGT, ALP activities and urea in serum and or urine were not affected by increases in HGA.
CONCLUSIONS: To avoid interferences by HGA in alkaptonuria concentration of HGA should be established before samples are assayed with peroxidase assays and benzethonium urine protein.