Definition
Plasma concentration is the concentration of a pharmaceutical agent in blood plasma at a given time after administration. Plasma is the liquid portion of blood excluding its cellular components. Concentration is typically expressed in µg/ml.
Plasma concentration-time curves
If plasma levels are measured several times after administration, a plasma concentration-time curve can be constructed from the values. The figure shows the idealized course after taking a tablet: the shape of the curve reflects the pharmacokinetics of the active ingredient. Influencing factors include:
- Dose
- Release
- Absorption rate
- Bioavailability
- Volume of distribution
- Elimination rate
Thus, for example, if the dose is increased, the maximum plasma concentration increases. From the measured values, various pharmacokinetic parameters can be obtained, with which the drug can be characterized. The curve can be calculated mathematically (e.g., Bateman function).
Maximum plasma concentration
Cmax is the highest plasma concentration reached after drug administration. It depends largely on the rate of absorption during peroral administration. The faster the drug enters the organism, the higher Cmax. The curve is shifted to the left. When a drug is administered with food, Cmax is often-but not always-reached later and the peak flattens. The curve shifts to the right. The time that elapses from application to maximum concentration is expressed as tmax. Drug-drug interactions can lead to an increase in Cmax and increase the risk for adverse effects.
Area under the curve (AUC).
The area under the curve (AUC) is always the same for intravenous administration, regardless of the rate and site of injection. It is directly proportional to the dose administered. It is usually lower for peroral administration because various barriers stand in the way of the drug. The bioavailability F is the quotient of the oral and intravenous AUC.
Other functions of plasma concentration
Plasma concentration indicates that a drug is absorbed and can exert systemic effects. A relationship to pharmacologic effects can often be established. However, the fact that an agent appears in the blood does not mean that it is pharmacologically active. For example, if it is not distributed to its drug target – such as behind the blood-brain barrier in the brain – it cannot exert its effects. Plasma concentration can be measured to monitor a therapy. For example, it is used to determine whether the concentration is within the therapeutic range. There is also an interest in plasma concentration, for example, because of drug-drug interactions or the influence of food on bioavailability (see above).
