Which of the following is used for the measurement of renal plasma flow?
AParaaminohippurate test
BPSP test
CHippuric acid test
DBSP test
Answer:
A. Paraaminohippurate test
Read Explanation:
Renal Plasma Flow (RPF) Measurement
- Para-aminohippuric acid (PAH) clearance is the gold standard method used to estimate effective renal plasma flow (ERPF).
- PAH is both filtered at the glomerulus and secreted by the proximal tubular cells, allowing for nearly complete removal from the plasma in a single pass through the kidneys.
- Fick Principle: The calculation is based on the principle that the amount of a substance entering the kidney equals the amount exiting the kidney via the renal vein and urine.
- The formula used is: RPF = (Urine Concentration of PAH × Urine Flow Rate) / Arterial Plasma Concentration of PAH.
- Inulin Clearance: While PAH is used for RPF, inulin clearance is the standard measurement for the Glomerular Filtration Rate (GFR) because it is freely filtered but neither reabsorbed nor secreted.
- Filtration Fraction (FF): This is the ratio of GFR to RPF (FF = GFR / RPF). Under normal physiological conditions, the filtration fraction is approximately 0.2 (or 20%), indicating that about 20% of the renal plasma flow is filtered into the Bowman's capsule.
- Note: In clinical practice, PAH clearance slightly underestimates true RPF because a small fraction of blood supplies non-excretory tissues (like the renal capsule and pelvic tissues) and therefore is not cleared of PAH.