The interference and elimination of nitrite on determination of total urinary protein by Pyrogallol Red–Molybdate method

Background

Some urine samples showed negative values of total urinary protein by Pyrogallol red–molybdate (PRM) method. Interestingly, these samples showed notably high levels of nitrite in the urine dipstick test.

Methods

A total of 120 urine samples were collected and categorized into four groups (0 - < 100, 100-<500, 500-<1000, ≥1000 mg/L) based on total urinary protein concentration. Various concentrations of nitrite (2, 10, 50, 100, and 200 mg/L) were added to urine samples to investigate potential interference of nitrite in total urinary protein measurement by the PRM method. Additionally, different concentrations of L-ascorbic acid (100, 500, 1000, and 2000 mg/L) were added to urine-nitrite mixtures to explore the possibility of reversing the interference effects.

Results

2 mg/L nitrite had no impact on the test results.10 mg/L nitrite only showed significant effects on the detection results among the groups with 0 - < 100 and 100-<500 mg/L (P < 0.05). 200 mg/L nitrite caused a noticeable decrease in the urinary protein detection results of the four groups (0 - < 100, 100-<500, 500-<1000, ≥1000 mg/L), and the concentrations were reduced to 2.7 %, 26.85 %, 75.22 %, and 89.33 % of their original levels, respectively. 500 mg/L L-ascorbic acid effectively eliminated the interference from 200 mg/L nitrite and almost had no effect on detection.

Conclusions

Nitrite had negative effect on the detection of total urinary protein by PRM method and L-ascorbic acid was effective in counteracting the interference caused by nitrite.