Creatinine normalization is a calculation, not a separate test, that adjusts a quantitative urine drug concentration for the patient's hydration. Because urinary drug levels swing with fluid intake, dividing the drug level by urinary creatinine removes most of that variability, allowing meaningful comparison of a single patient's results over time.¹

A raw urine drug level can rise or fall simply because a patient drank more or less water, which can mimic changes in use. Normalizing to creatinine, a steady byproduct of muscle metabolism excreted at a fairly constant rate, corrects for hydration so a real change in drug excretion can be distinguished from a dilution artifact.²

The laboratory divides the measured drug concentration by urinary creatinine to yield a normalized value for comparison across a patient's serial samples. Normalized values are not calculated when creatinine is below 20 mg/dL, because such dilute samples are considered invalid for this purpose.¹ A common application is distinguishing new cannabis use from residual THC excretion in chronic users by comparing normalized cTHC over time.¹

• Use creatinine normalization for serial, same-patient comparison, not single results.²
• Do not normalize when creatinine is below 20 mg/dL (dilute, invalid sample).¹
• Apply it to distinguish new use from residual excretion (e.g., THC) over time.¹
• Remember it does not correct for metabolism, timing, interactions, or body habitus.²
• Consult a toxicologist for complex serial interpretations.