Bioavailability of a drug

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

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)