Substance conversion

When concentration data at the level of measured substances have to be converted to concentration data at the level of active substances, then substance conversions should be specified to provide the rules for deriving active or inactive substance concentrations from measured substance concentrations. This section will first describe the basic substance conversion, and following this the way in which substance authorisations can be used to refine these calculations.

For each measured substance in the concentration data, there may be zero or more conversion rules (records in the data source), each linking to an active or inactive substance. Note that the substance conversion procedure has been implemented for two cases:

  1. Measured substances link to one or more exclusive substances which are possible translations (i.e. the measured concentration is assumed to be the sum of concentrations for all linked substances, but it is assumed that only one substance is present in the sample, therefore the measured substance is considered to be one of the linked substances);

  2. Measured substances link to one or more exclusive substances plus one (non-exclusive) substance that is a metabolite of the others. The metabolite can occur together with any of the exclusive substances.

It is assumed that either all conversion rules linked to a measured substance are marked as exclusive (case 1), or precisely one rule is marked as exclusive and the other rules are marked as not exclusive (case 2). If this is not the case for any set of rules linked to a measured substance, then this is regarded as erroneous data.

For each measured substance concentration measurement on a food sample (positive or non-detect), the active substance concentration allocation is done using the following procedure:

  1. If there is no conversion rule available for the measured substance linked to any active substance of interest, then no conversion is required. The measured substance is ignored in the active substance concentrations set, unless it is an active substance itself.

  2. If there are conversion rules available for the measured substance, then concentrations (positive or non-detect) are converted from the measured substance to one or more linked substances as specified by the conversion rules. Two substance allocation methods are available, one of which should be selected:

Tier 1) Most potent: For each measured substance, the linked substances are restricted to the active substances of interest. The concentration of the measured substance is assigned to the most potent active substance in this set. Potency is specified by the relative potency factors. All other candidate active substances are assigned a zero concentration. I.e., the measured substance concentration is allocated for 100% to the most potent substance specified by the conversion rules and for this allocation, the concentration or LOR is multiplied by the molecular weight correction factor.

Tier 2) Random: One of the conversion rules is drawn randomly (with equal probability), including the rules of both active and other substances. This drawn rule is used as follows to generate active substance concentrations:

  • If the drawn conversion rule is marked as exclusive, the concentration or LOR is allocated to the linked substance.

  • If the drawn conversion rule is marked as not exclusive, a proportion p, specified by the drawn conversion rule, of the concentration or LOR is allocated to the linked substance. The remaining proportion (1-p) is allocated to one other substance, which is the substance that is linked to the measured substance in a conversion rule marked as exclusive (in this case it is assumed that precisely one record per measured substance is marked as exclusive).

In Tier 2 all assigned concentrations are multiplied by the molecular weight correction factor. All unselected candidate substances are assigned a zero concentration. After allocation, the resulting set of substance concentrations is restricted to contain only concentrations for the active substances of interest.

(Included for research purposes) Nominal: The substances specified through the conversion rules are allocated a nominal value based on all possible conversion rules. This may be regarded as the nominal or average allocation value of the random sampling method.

  • All conversion rules are marked as exclusive: The measured substance concentration is divided over all n active substances specified with equal proportions 1/n, accounting for the molecular weight correction factor for all substances.

  • Precisely one conversion rule is marked exclusive and n conversion rules are marked as not exclusive: The measured substance concentration is divided over all active substances specified, with a proportion 1/2 + 1/n for the substance belonging to the exclusive conversion rule, and equal proportions 1/n for the other substances, accounting for the molecular weight correction factor for all substances.

Use of substance authorisations in substance conversion

When substance authorisations are available, then these can be used to exclude conversions of measured substances to unauthorised substances on a given food. The information is used as follows in the substance conversion procedures:

Option 1) Most potent: The set of candidate active substances from which the most potent active substance is to be drawn is reduced to only the substances with authorised uses. However, if none of the candidate active substances is authorised, then the most potent of the unauthorised substances is selected for active substance allocation.

Option 2) Random: The set of conversion rules from which to draw is reduced to the rules linking to authorised substances or the non-exclusive substance (thus allowing the selection of a possibly unauthorised metabolite of an authorised substance). If none of the conversion rules links to an authorised substance, then one rule is drawn from the full set of all (unauthorised) conversion rules.

Nominal: The set of conversion rules is reduced in the same way as in Tier 2. Nominal calculation is performed on the resulting set of conversion rules.