Topic: Organ function tests-liver, kidney, thyroid
Question: Define clearance? Enumerate the methods available for its determination.
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In pharmacokinetics, clearance is a term used to describe the body’s ability to eliminate a drug. It’s defined as the volume of plasma from which a drug is completely removed per unit time. Clearance is usually expressed in units of volume per time, such as milliliters per minute (mL/min) or liters per hour (L/h).
There are several methods available for the determination of clearance, including:
- Plasma Clearance: This is the most common method used to determine clearance. It involves measuring the concentration of a drug in the plasma at various time points after administration and then using these measurements to calculate the rate at which the drug is being eliminated from the body.
- Renal Clearance: This method specifically measures the rate at which a drug is eliminated by the kidneys. It involves collecting urine over a specific period of time and measuring the concentration of the drug in both the urine and the plasma. The renal clearance can then be calculated using these measurements.
- Hepatic Clearance: This method measures the rate at which a drug is metabolized by the liver. It’s more complex to determine than plasma or renal clearance because it requires knowledge of the drug’s hepatic extraction ratio and the liver’s blood flow rate.
- Non-Renal Clearance: This method measures the rate at which a drug is eliminated by all routes other than the kidneys, such as metabolism in the liver or excretion in the bile. It’s calculated by subtracting the renal clearance from the total plasma clearance.
- Salivary Clearance: This method measures the rate at which a drug appears in saliva. It’s a non-invasive method that can be used to estimate the plasma clearance of certain drugs.
- Lung Clearance: This method measures the rate at which a drug is eliminated by the lungs. It’s primarily used for drugs that are inhaled or that are known to be excreted in the breath.
Each of these methods has its own advantages and limitations, and the choice of method depends on various factors, including the properties of the drug, the route of administration, and the information that’s needed.
