Timed collected specimens may be more representative. In routine urinary biomonitoring spot samples adjusted for dilution (by creatinine or relative density) are adequate alternatives. Highly diluted or very concentrated urine (creatinine: 3.0g/litre; relative density: 1.030) are not suitable for such adjustment, thus new specimens should be collected.
Most absorbed substance and active metabolites can be found in blood, which is the second most common biological matrix used in routine biomonitoring. Unlike urine, composition of blood is regulated within narrow limits (e.g. pH, concentrations of natural substances), thus it seldom needs adjustment. Blood sampling is useful for inorganic chemicals (e.g. metals) and for organic chemicals that are poorly metabolised and have a sufficiently long half-life.