Drug concentration in blood plasma

2021-06-24 02:51 PM

The plasma drug concentration/area under curve describes a patient's plasma drug concentration at various time points after drug administration.

Significance of drug concentration in plasma

The plasma drug concentration reflects the amount of drug that is present in the plasma at a given time. Plasma drug concentrations can be measured by suitable methods.

It is not easy to determine the drug concentration in the tissue, (eg, in the lung tissue in the treatment of pneumonia), with drugs that require high concentrations in the target organs when administering the drug. A correct dose of drug concentration in plasma will help us in treatment. For example, the use of antibiotics requires sufficient doses to achieve minimum inhibitory plasma concentrations (MIC is the minimum effective concentration).

Drug concentration in plasma/area under the curve

The plasma drug concentration/area under the curve (Cp/AUC) describes a patient's plasma drug concentration at various time points after drug administration. The plasma drug concentration/area under the curve for a single dose oral drug depicts a curve that rises (according to the amount of drug absorbed) and reaches peak concentration (maximum concentration), then downward curve (expression of drug being eliminated).

Figure: Area under the curve

Maximum drug concentration (Cmax), time to reach maximum drug concentration (Tmax), and area under the curve (AUC).

Figure: Effect time

AUC, Cmax and Tmax are parameters used to evaluate drug efficacy. The effectiveness of the drug depends on the pharmacokinetic and pharmacological properties of each drug class.

For example, the effectiveness of aminoglycosides (amikacin, gentamicin, kanamycin) depends on Cmax; while the effectiveness of beta-lactam drugs (ampicillin, amoxicillin, cephalexin...) depends on the duration of high plasma drug concentrations above the MIC. Therefore, the total daily dose of aminoglycosides should be administered once, preferably (except in pregnant women and treating endocarditis) to achieve higher blood concentrations instead of taking 2 to 3 times. /day. The total daily dose of beta-lactam drugs for adults should be divided into 4 doses to maximize the time bacteria are exposed to antibiotics.

Factors determining the evolution of the drug concentration curve

ADME factors (Absorption, Distribution, Metabolism, Elimination - absorption, distribution, metabolism, elimination) determine the evolution of the drug concentration curve. Altered ADME factors may affect plasma drug concentrations. For example, the plasma drug concentration curve will increase if: 

Strong absorption.

Low distribution.

Little conversion.

Slow elimination.

On average, ADME factors will be different such as in patients with kidney disease, the ability to eliminate aminoglycosides (such as amikacin, gentamicin...) will be low and the drug concentration in the blood plasma will increase, leading to a risk of the toxic organism. The dose of aminoglycosides should be adjusted for this patient to prevent the risk of ototoxicity and nephrotoxicity.

Determination of drug plasma concentrations in a specific patient 

Plasma drug concentrations can be determined by biochemical methods; however, this is not yet possible in many health facilities. Moreover, each concentration measurement gives only one point of the whole curve, so it is difficult to guess without extensive training and experience.

In practice, clinical manifestations of beneficial or toxic effects of a drug should be sought by close observation when monitoring patients clinically.