Why urine dilution matters

Two urine samples may show different biomarker concentrations simply because one is more dilute. Fluid intake and losses through sweating, among other factors, influence dilution. Expressing selected results relative to urinary creatinine aims to reduce part of this variability, so comparison does not depend only on the amount of water present. It is an adjustment for interpretation, not a new independent measurement of the worker’s exposure.

What changes in the reported result

A concentration per litre of urine and an amount per gram of creatinine are different quantities. They cannot be compared directly against the same numerical reference. For example, micrograms per litre are not equivalent to micrograms per gram of creatinine. Reports should retain the analyte, unit, method and reference used, preventing inappropriate conversions or misleading conclusions.

Not every biomarker should be adjusted

The usefulness of correction depends on the substance’s elimination pathway and the evidence available for the biomarker. For some compounds, correction is inappropriate. Follow the validated method and the unit used by the current interpretation value. Applying the adjustment automatically to all results can introduce distortion, even if it produces a table that appears more consistent or easier to compare.

Individual influences

Creatinine is related to muscle mass and renal function, among other variables. Correction therefore does not make samples from all people perfectly comparable. Clinicians need relevant contextual information and should assess unusually dilute or concentrated samples. Deliberately changing hydration to force a result is inappropriate, and this isolated measurement should not be used to diagnose kidney disease.

Quality begins before laboratory analysis

Collection timing, containers, identification, transport and possible contamination remain important. Creatinine correction cannot repair a sample collected at the wrong time or contaminated by the person’s hands. In biological exposure monitoring, traceability should establish what was collected, when and under which working conditions, allowing the laboratory result to be connected with the actual exposure period.

Using results for follow-up

Before comparing campaigns, check that the biomarker, units and sampling time are equivalent. If the laboratory or reporting format changes, establish comparability before interpreting a trend. Results must be considered alongside exposure through all relevant routes and preventive assessment. A corrected figure alone is not a statement that the workplace is safe, nor proof that existing controls are adequate.