Objective:To compare the differences in the results of 24-hour urine citric acid, oxalic acid, and phosphoric acid detection between thymol and concentrated hydrochloric acid preserved urine.Methods:Urine samples were collected from 50 patients with upper urinary tract stones who were admitted to the Second Affiliated Hospital of Zhengzhou University from July 2023 to August 2023. There were 38 males and 12 females, with the age of (45.52 ± 13.68) years old. On the second day of admission, patients were required to empty their bladders in the morning and collect urine over a 24-hour period. The urine was evenly divided and stored in two sealed plastic containers. In one container, concentrated hydrochloric acid (15 ml, concentration of 6 mmol/L) was added as a preservative, forming the hydrochloric acid group. In the other container, 2 g of thymol was added as a preservative, forming the thymol group. The concentration of oxalic acid, citric acid, and phosphoric acid in urine were measured, and the measurement results were multiplied by the total amount of urine to obtain the final measurement results. The differences in the measurement results of citric acid, oxalic acid, and phosphoric acid between two groups were compared.Results:In the concentrated hydrochloric acid group, the median value of citric acid was 160.87(95.37, 245.61) mg/24h, the median value of oxalic acid was 33.09(26.32, 39.37) mg/24h, and the mean value of phosphoric acid was (731.06 ± 240.62) mg/24h. In the thymol group, the median value of citric acid was 340.40(166.56, 561.77) mg/24h, the median value of oxalic acid was 33.91(26.51, 44.56) mg/24h, and the mean value of phosphoric acid was (759.07 ± 273.23) mg/24h. Compared with the thymol group, the citric acid level in the concentrated hydrochloric acid group was significantly lower ( Z=4.31, P< 0.001), but there was no significant difference in oxalic acid ( Z=0.85, P=0.393) and phosphoric acid ( t=1.65, P=0.106) between the two groups. Conclusions:Compared with thymol, the results of 24 h urine citric acid with concentrated hydrochloric acid as preservative is lower, but there is no significant difference between oxalic acid and phosphoric acid.The use of thymol as a preservative may be more suitable than concentrated hydrochloric acid for the detection of citric acid, oxalic acid and phosphoric acid in 24-hour urine metabolism analysis.