Drug levels in blood

DRUG LEVELS IN BLOOD
Department of Pharmacy Practice
1

Drug Levels In Blood
Drug response relationships:-
 Dose Response
 Time Response

A- Which drug has the lower threshold
B- Which has the greater maximum effect

• The main objective of administration of the optimal drug therapy is to provide
maximum therapeutic effect and minimum toxic effect
• Both effects depends on the concentration of the drug at the site of action
Owing to its difficultly in direct measurement, plasma concentration can
consider instead of drug concentration at site of action.
• As a direct relationship exists between the concentration of drug at the bio phase
(site of action) and the concentration of drug in plasma.
• The rate and extent of the drug absorption is explained by using plasma drug
concentration – time profile
• These profiles are obtained by measuring the concentration of the drug in
plasma sample taken at various time intervals after administration of dosage
form and plotting the graph by taking concentration of drug in plasma on y axis
and time on x axis

Two categories of parameters can be evaluated from plasma
concentration - time profile:-
• PHARMACOKINETIC PARAMETERS:
i. Peak plasma concentration (CMAX )
ii. Time of peak plasma concentration(TMAX )
iii. Area under curve (AUC)
• PHARMACODYNAMIC PARAMETERS:
i. Minimum effective concentration (MEC)
ii. Maximum safe concentration (MSC)
iii. Onset of action
iv. Onset time
v. Duration of action
vi. Intensity of action
vii. Therapeutic range
viii. Therapeutic index
ix. Termination of action

PHARMACOKINETIC PARAMETERS:
i. Peak plasma concentration: Also called as peak height
concentration /Maximum drug concentration.
 The point at which max drug concentration in plasma – PEAK
and the concentration of drug at peak – PEAK PLASMA
CONCENTRATION
 At CMAX ,rate of absorption = rate of elimination.
 It depends on dose administered , rate of absorption, rate of
elimination .Units are µg/mL
ii. Time of peak concentration: The time for drug to reach peak
concentration in plasma is t-max
 It is imp parameter for estimating the rate of absorption
 Onset of action and Onset time is dependent on t-max
 Units for t-max is hours

Area under the curve: It is the total integrated area under the plasma
drug concentration- time curve which expresses the extent of
drug absorption after its administration i.e, amt of drug that
comes into the systemic circulation.
 Units for AUC is µg/ml. hr
It is imp parameter in estimating the bioavailability of the drug as it
represents the extent of absorption and also in assessing the efficacy
of multiple dose drugs used in treatment of chronic diseases
PHARMACODYNAMIC PARAMETERS:
i. Minimum effective concentration: It is the mini concentration of
drug that is required to show the therapeutic response.
 Also referred as Minimum inhibitory concentration (MIC)
 MIC term is generally used in case of antibiotics and it describes
the minimum concentration of antibiotic in plasma required to
kill or inhibit the growth of micro-organisms.
 Drug concentration below MEC is Sub-therapeutic level

ii. Maximum safe concentration(MSC) :- It is the concentration
of drug in plasma above adverse effects or toxic effects
are observed
 It is also called as mini toxic concentration (MTC)
iii. Onset of action: When plasma drug concentration just
exceeds the required MEC, the pharmacological response
starts and this is called as onset of action.
 It depends on dose administered, rate of drug release from
its dosage form, rate of absorption and rate of elimination
of the drug
iv. Onset of time: It is the time required by the drug to start
producing pharmacological response.
It corresponds to the time for the plasma concentration to
reach minimum effective concentration (MEC) after
administration of drug

v. Duration of action: The time period for which the plasma
concentration of drug remains above the MEC level is known as
duration of (drug) action.
It can also be defined as the difference between onset time and
time for the drug to drop back to MEC.
vi. Intensity of action: It is the maximum pharmacological response
produced by the peak plasma concentration of drug.
Also called as peak response
vii. Therapeutic range: The concentration of drug between minimum
effective concentration and maximum safe concentration is known as
therapeutic range.

viii. Therapeutic index: The ratio of MSC and MEC is called
Therapeutic index
Also defined as the ratio of dose required to produce toxic or
lethal effects to dose required to produce therapeutic effects
ix. Termination of action: The time period after which the drug
levels in the blood decline below MEC
No therapeutic effect is observed

APPLICATIONS:
1. It is used in investigating:
 The extent of drug absorbed
 Factors interfering absorption
 Nature of dose response plots
 Effect of ROA
 Optimum dosage regimen
 Elimination of the drug
 Factors affecting elimination
 Relationship between drug concentration and pharmacological
response.
 Distribution of drugs.

2. It allows adjustment of dosage regimen for individualizing and
optimizing the drug therapy
3. Helps in monitoring the progress of the treatment in the
presence of any altered physiological functions which helps in
modification of dosage
4. Helps in therapeutic drug monitoring for drugs with narrow
therapeutic index
5. Helps in correlating the plasma drug concentration with
pharmacological response.

DISADVANTAGES:
1. Since drug concentration in plasma is usually low , HPLC/
Liquid chromatography mass spectroscopy is required for
drug analysis
2. Blood sample is collected by using syringe under medical
supervision and patient may not accept for repeated
withdrawal of blood
3. In case of pediatrics and geriatrics ,it is difficult to obtain
blood samples. In such cases urine sampling is the alternative.

Drug levels in blood

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