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.