Bio Transfer Factor (BTF) is a constant to describe the relationship between chemical substance in a tissue and the rate of ingestion to body of particular toxic agent (chemical substance). Often, the concentration forms a linear or straight line as the rate of chemical ingestion inreases as well. Thus at this case, the BTF is constant. I will make this simpler, BTF is a ratio between the amount of chemical per amount of body weight (mg A/kg_tissue) and the rate of ingestion of chemical entering the body (mg A/day. If you do not follow previous post, you are probably unfamiliar with A, A is representing toxic chemicals. I suggest you read the example about physiologically based pharmacokinetics, part 1, part 2 and part 3, especially this example is related to those examples as well.

Concentration of chemical (mg A/kg_body) = BTF x dose (mg A/day)

Continuing previous example, suppose we would like to know the BTF of each organs, blood, kidneys and liver, it would be easily computed.

BTF = C (mg A/kg_body) / dose (mg A/day)

BTF_blood = C_blood / dose (mg A/day) = 0.81 mg A/kg_blood / 0.1 mg A/day = 8.1 (mg A/kg_blood)/(mg A/day)

BTF_kidneys = C_kidneys / dose (mg A/day) = 3.2 mg A/kg_kidneys / 0.1 mg A/day = 32 (mg A/kg_kidneys)/(mg A/day)

BTF_liver = C_liver / dose (mg A/day) = 8.1 mg A/kg_liver / 0.1 mg A/day = 81 (mg A/kg_liver)/(mg A/day)

It is very useful to know BTF, because from BTF, assumed that BTF is independence with the ingestion rate, we can use it for different ingestion rate. Suppose we would like to know the concentration of A in blood, kidneys and liver if we fed the rats with 0.2 mg/day. We could easily calculate :

C_blood = BTF_blood x dose (mg A/day) = 8.1 (mg A/kg_blood)/(mg A/day) x 0.2 mg A/day = 1.61 mg A/kg_blood

C_kidneys = BTF_kidneys x dose (mg A/day) = 32 (mg A/kg_kidneys)/(mg A/day) x 0.2 mg A/day = 6.4 mg A/kg_kidneys

C_liver = BTF_liver x dose (mg A/day) = 81 (mg A/kg_liver)/(mg A/day) x 0.2 mg A/day = 16.2 mg A/kg_liver

So in this series post, you have learned about the principle theory of physiologically based pharmacokinetics, you also have learned simple calculation about the pathways of chemicals inside our digestion process, how many percent is absorbed and urinated, and then followed by how to calculate the concentration of chemical inside your body, simple and a bit advanced one. Since I am Environmental Engineer, and I also deal with water treatment, it must be noted that concentration in this post and toxicology related, is not mg/L unit instead mg chemical/kg_{body tissue}.

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