Determination of Total Urate-Like Chromogens (TUEC)
Add 100 μL of serum to 2.5 ml of sodium hydroxide 0.4 mol/L and leave for 30 minutes at room temperature (20–25°C), then add 0.5 ml of sodium carbonate 1 mol/L and 1 ml of the diluted phosphotugstic acid reagent and mix. After 10 minutes read the absorbance at 700 nm against water as blank.
Calibration Curve
The calibration curve was prepared by diluting the stock standard with water to have the working standards, 2.9745, 1.983, .... and 0.5949 mmol/L respectively.
100 μL of each working standard solution were added to 2.5 ml of the 0.4 mol/L sodium hydroxide solution, then very rapidly, 0.5 ml of sodium carbonate 1 mol/L, and 1 ml of the phosphotungstic acid working reagent were added and mixed. After 10 minutes, the absorbance was measured at 700 nm against water as blank.
Because of the destructive effect of sodium hydroxide solution on uric acid, the calibration curve is linear only in the small concentrations (up to 2.9745 mmol/L). Meanwhile, the measured TUEC in serum is linear up to 5.576 mmol/L (the highest value recorded).
The linear regression equation Y = 0.0185 + 0.21196 X, the correlation coefficient r = 0.9975, Sy.x = 0.0075.
Precision
Table 2 illustrates the within-run and between run data. Ten repeated estimates of serum pools were performed daily for six days.
The Cvs ranged from 0.23 to 2.4% over a concentration range from 2.513 to 4.995 mmol/L.
Interference from drugs
It has been reported that L-DOPA administration produces spuriously high values for serum urate as measured by the commonly used phosphotungstate reduction methods [1]. Three of its major metabolites; dopamine, 4,4-dihydroxyphenylacetic acid, and 3-methoxyphenylacetic acid also have the same effect [2]. Moreover, we found that VELOSEF, a semisynthetic cephalosporine antibiotic, administration gives total chromogens value as high as 5.576 mmol/L compared with 2.38 mmol/L before administration. Thus before assay, a careful history must be taken to determine if one of these drugs has been ingested recently by patient.