Analysis of interference effect of nitrites on total urinary protein assay by pyrogallol red molybdenum method
Objective To investigate the interference effect of nitrite on the determination of 24 h urine total protein by pyrogallol red method and evaluate the feasibility of using vitamin C to correct this interference.Methods According to the CLSI EP7-A3 document,fresh urine specimens with 24 h urine total protein concentrations of 150 mg/L,500 mg/L and 1 000 mg/L were selected as control samples,and interfering samples containing different concentrations of sodium nitrite were prepared.The interference of nitrite was con-firmed through paired difference experiments,and the relationship between nitrite concentration and interference level was clarified u-sing dose-effect experiments.The effects of different concentrations of vitamin C on correcting the interference caused by 200 μg/mL so-dium nitrite were evaluated.Additionally,61 nitrite-positive and 40 nitrite-negative clinical specimens were collected to compare the relative differences before and after vitamin C correction to assess its clinical application value.Results In the paired difference exper-iments,200 μg/mL sodium nitrite had relative interferences of-157.8％and-36.2％on the determination of urine total protein of 150 mg/L and 500 mg/L respectively,which were significantly greater than 1/2TEa(22％).Although there was a-20.5％negative interference for the high concentration of 1 000 mg/L urine total protein,the interference was within the acceptable range.The dose-effect experiment results showed that as the nitrite concentration in urine increased,the negative interference on urine total protein de-tection also gradually increased.In the presence of 200 μg/mL sodium nitrite as the final concentration,the addition of 0.2 mg/mL vi-tamin C corrected the urine total protein at 150 mg/L and 500 mg/L to 148 mg/L(-1.1％)and 402 mg/L(-19.5％),respectively,both within the acceptable range.The relative differences produced by vitamin C correction in the nitrite-positive clinical specimen group were significantly higher than those in the nitrite-negative group(P＜0.01).Conclusions Nitrite produced negative interference on the determination of urine total protein by the pyrogallol red method,especially when the urine total protein is 150 mg/L,which needs attention of clinical laboratories.The addition of 0.2 mg/mL vitamin C could effectively correct the interference of nitrite in low-concentration of urine total protein specimens,showing potential clinical application value.
24 h urine total proteinnitriteinterference analysisvitamin Ccorrection test
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