Biological Monitoring Calculator

A spot urine sample's dilution varies with how much fluid the worker has consumed, when they last urinated, and time of day, so the same underlying exposure can produce a more concentrated or more dilute urine sample depending on hydration alone — creatinine, a relatively stable byproduct of muscle metabolism excreted at a fairly constant rate, is used to correct for that variability. Blood specimens don't have this dilution problem in the same way, since blood volume and composition are physiologically regulated within a much narrower range, so blood-based biological monitoring results are typically interpreted directly without a creatinine adjustment. Because the corrected urine result comes out in µg per gram of creatinine, the BEI reference value you enter for a urine specimen needs to already be in those same units (µg/g creatinine) — real ACGIH urine BEIs are commonly published this way — otherwise the ratio compares mismatched units and the result is meaningless.

A urine creatinine concentration that's too low generally indicates an overly dilute sample (the worker may have consumed a lot of fluid before providing the sample), while a concentration that's too high suggests an overly concentrated sample — both extremes are commonly considered less reliable for biological monitoring interpretation, since the creatinine-correction math becomes less trustworthy at those extremes. Guidance in this range, roughly 0.3 to 3.0 grams of creatinine per liter, is widely referenced in industrial hygiene biological monitoring practice, though a sample flagged outside this range doesn't necessarily mean the exposure result is wrong — it means the result should be interpreted cautiously or the sample recollected if practical.

A BEI ratio of 1.0 means the worker's (creatinine-corrected, for urine) measured value exactly equals the published Biological Exposure Index reference value for that substance — the BEI itself represents a level that ACGIH associates with the majority of healthy workers exposed at or below the corresponding occupational exposure limit, not a hard toxicological safety threshold. A ratio above 1.0 indicates the result exceeds that reference point and generally warrants investigation into exposure sources and controls, but it isn't automatically evidence of an adverse health effect.

This calculator computes its BEI ratio, action tier, and trend from a single measured value regardless of how many workers you enter in the group field — group size is provided for context only and doesn't feed into the calculation shown. Group-level biological monitoring interpretation, such as comparing a geometric mean across multiple workers to a BEI, requires aggregating each individual's result separately rather than treating one worker's single measurement as representative of the whole group.