9(a). A person's glomerular filtration rate (GFR) provides an indication of the health of their kidneys. The GFR is a measure of the volume of blood that can be filtered by the kidneys every minute

GFR can be estimated by monitoring the blood concentration of creatinine, which is a breakdown product of

creatine phosphate in muscles.

(1) Suggest two characteristics of a patient that must be taken into account when using this GFR measurement

to diagnose kidney damage.

Explain why each characteristic must be considered.