1) Plasma Protein Binding
Only free (unbound) drug is filtered. High protein binding may reduce filtration but can still be secreted via active transport.
2) Urine pH and Drug Ionization
Weak acids and bases can become more ionized in certain pH environments (ion trapping).
For instance, alkalinizing the urine (increasing pH) promotes excretion of weak acids by reducing their reabsorption.
3) Renal Blood Flow / GFR
Decreased blood flow (e.g., shock, heart failure) or kidney disease lowers GFR, reducing filtration and slowing drug elimination.
4) Active Secretion
Saturation or inhibition of transporters can reduce renal secretion.
Certain drugs compete for the same transporter, affecting each other’s excretion.
5) Age & Disease
Neonates and the elderly often have reduced renal function.
Chronic kidney disease (CKD) reduces excretion capacity.
6) Drug Interactions
Some drugs can compete for the same tubular secretion transporters (e.g., probenecid can block the secretion of penicillin).
