PAH (para-aminohippurate) measurement is used to determine renal plasma flow, a key indicator of kidney function.
Understanding Renal Plasma Flow and PAH
Renal plasma flow (RPF) is the volume of plasma that flows through the kidneys per unit of time. Measuring RPF helps assess how well the kidneys are filtering blood. PAH is particularly useful for this purpose because of its unique characteristics:
- Complete Excretion: PAH is almost completely cleared from the renal plasma as it passes through the kidneys. This means that nearly all PAH entering the kidneys is excreted in the urine.
- Calculation of RPF: By measuring the concentration of PAH in the urine and plasma, the effective renal plasma flow (ERPF) can be calculated. ERPF is a close estimate of the actual RPF.
How PAH Clearance Works
The process involves injecting a known amount of PAH into the bloodstream and then measuring its concentration in both the plasma and the urine. The rate at which PAH is cleared from the plasma and appears in the urine provides a direct measure of the renal plasma flow. The formula used to calculate ERPF is:
ERPF = (Urine Concentration of PAH x Urine Flow Rate) / Plasma Concentration of PAH
Clinical Significance
Measuring renal plasma flow using PAH clearance is valuable in diagnosing and monitoring kidney diseases. Abnormal RPF values can indicate conditions such as:
- Renal Artery Stenosis: Narrowing of the renal arteries, which reduces blood flow to the kidneys.
- Glomerulonephritis: Inflammation of the kidney's filtering units (glomeruli).
- Acute Kidney Injury (AKI): A sudden loss of kidney function.
- Chronic Kidney Disease (CKD): A progressive loss of kidney function over time.
