All of the following statements regarding bioavailability of a drug are true
except:
1. It is the proportion (fraction) of unchanged drug that reaches the
systemic circulation.
2. Bioavailability of an orally administered drug can be calculated by comparing
the Area Under Curve (0- ) after oral and intravenous (iv) administration.
3.Low oral bioavailability always and necessarily mean poor absorption.
4.Bioavailability can be determined from plasma concentration or urinary
excretion data.
Answer
3.Low oral bioavailability always and necessarily mean poor absorption.
Reference:
Katzung, 7th Edition Page 40
KDT 5th Edition Page 15 & 4th Edition Page 15 Figure 2.4
Quality
Reader
Status
New
QTDF
KDT
Discussion
Bioavailability of a drug is defined as the fraction of unchanged drug reaching
the systemic circulation following administration by any route.
Following intravenous route the bioavailablilty is 100 %.
Oral Route is associated with low bioavailability due to the following reasons
1. The drug may be incompletely absorbed
2. The absorbed drug may under go first pass metabolism in intestinal wall/liver
or may be excreted in bile
IM/SC Route may be associated with a low bioavailability and that is due to
1. Local Binding
Bio availability is determined by the Area under the Plasma Concentration-time
Curve.
Explanation
1. It is the proportion (fraction) of unchanged drug that reaches the systemic
circulation.
2. Bioavailability of an orally administered drug can be calculated by comparing
the Area Under Curve (0- ) after oral and intravenous (iv) administration.
3.Low oral bioavailability always and necessarily does not mean poor absorption.
It could be also due to first pass metabolism
4.Bioavailability can be determined from plasma concentration or urinary
excretion data.
Comments
A drug with low bioavailability will have a very high Oral Dose compared to its
parenteral dose (eg Propanolol)
Tips
This is an area from which few problems may be asked
Source: RxPG AIPG 2003 Book by TargetPG, Author : Dr.J.Mariano Anto Bruno
Mascarenhas
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