Creatinine normalization approach to diluted urine samples screened by LC-MS/MS method

INTRODUCTION: Urine is the most used matrix in drug analysis; however, it is susceptible to adulteration or tampering. Urine creatinine is the most important urine integrity parameter used as an indicator of dilution. This study aimed to evaluate the prevalence of diluted urine samples and the change in positivity after creatinine normalization.
METHODS: Urine samples screened by the LC-MS/MS method over a 3.5-year period (n=21,927) were included in the study. Positivity rates were evaluated in both total and diluted urine samples. Additionally, the impact of creatinine normalization on samples with substance concentrations above the limit of quantitation (LOQ) and below the cut-off was investigated.
RESULTS: A total of 350,832 tests were conducted on 21,927 urine samples, resulting in an overall positivity rate of 21.2% (n=4652). The ratio of diluted urine was 1.6% (n=343), with 61.5% (n=211) testing negative (<LOQ), 23.3% (n=80) testing positive (at least one substance >cut-off), and 15.2% (n=52) testing above LOQ and below cut-off. After creatinine normalization in diluted urines, the sample positivity rate increased from 23.3% (n=80) to 33.8% (n=116) (p<0.001), and the substance positivity rate increased from 2.3% (n=125) to 3.9% (n=212) (p<0.001).
DISCUSSION AND CONCLUSION: Precautions should be taken in reporting diluted urine samples to avoid reporting false negative results. The creatinine normalization approach shows promise in laboratories using quantitative screening methods such as LC-MS/MS for samples with substance concentrations above the LOQ and below the cut-off. However, more clinical and laboratory collaboration is needed for its routine application.