Bioavailability and bioequivalence

By extending the definition of bioavailability, this chapter could be seen to answer Section B(ii) of the 2023 CICM Primary Syllabus which expects the exam candidate to "Describe the absorption of drugs and factors that influence this". Absorption and bioavailability are related concepts. 

This has come up three times in the historical CICM Part I papers:

In the earlier question from  2010,  candidates were asked to "describe the factors that affect the fraction of drug reaching the systemic circulation", specifically by the oral route. This 80% part of the question was followed by a 20% section asking about the effect of shock on these factors. The overall concept of bioavailability was therefore only partly touched upon - the college were mainly looking for factors which influence gastrointestinal drug absorption, something explored in a separate chapter. However absorption is not necessarily always oral, shock has an effect on bioavailability from all sites, and the definition of bioavailability needs to be explored in order for fundamental issues to be well understood. 

Fortunately, there's not much to understand. In summary:

  • Bioavailability is the fraction of the dose which reaches systemic circulation intact
  • IV bioavailability is by definition 100%
  • "Absolute" bioavailability compares one non-IV route with IV administration. 
  • "Relative" bioavailability compares one non-IV route or formulation with another (instead of using IV route as a reference).
  • Bioavailability is measured using the area under the concentration-time curve (Dost's Law). The ratio of AUCs is the bioavailability value.

As far as published literature goes, one would probably need to be familiar with the College-assigned text (Birkett et al, 2009). For the purpose of passing the exam Chapter 5 (p.45-50) from Pharmacokinetics Made Easy is probably an acceptable minimum. Additional information of interest (which would not be essential) could be found in Aldo Rescigno's 1991 article "Bioequivalence". Jan Koch-Weser's ancient NEJM article (1971) is paywalled, but is interesting if you can get a hold of it. For the people who do not want to pay for a copy of Birkett, at the time of writing ResearchGate seems to have a copy of the excellent review by Allam et al (2011). The pharmacokinetic effects of shock (with a specific focus on sepsis) is discussed by De Paepe et al (2002). The influence of critical illness on pharmacokinetics had come up in the Fellowship Exam papers (eg. Question 5 from the first paper of 2011) and a brief revision chapter exists in the CICM Part II  Required Reading section on pharmacology and toxicology.

Definition of bioavailability

 In their comments to  Question 5(p.2) from the first paper of 2010, the college mention that "for a good answer candidates were expected to define bioavailability". This would have been easy, as the question itself essentially paraphrases a definition offered by  Pharmacokinetics Made Easy (Birkett, 2009 -the new foundation text for the 2017 CICM syllabus). To quote Birkett:

"Bioavailability is the fraction of the dose which reaches systemic circulation as intact drug"

Other definitions are possible and perhaps even desirable, but exploring them is not recommended for exam candidates. These come from sources not listed as required reading texts, and should therefore be viewed cautiously as apocryphal and possibly heretical.  The grimoire of Goodman & Gilman (12th ed) defines bioavailability as "the fraction of the dose (F) that is absorbed and escapes any first-pass elimination", focusing largely on oral administration. Rowland and Tozer define it as "as the fraction, or percentage, of the administered dose absorbed intact".  

The flaw of most definitions is the tendency to ignore the possibility that drugs might get absorbed by other routes, and that serum concentration may not be the concentration you are most interested in.  The United States FDA made a valiant effort to distance itself from the oral fixation of other authors by defining it as "the rate and extent to which the active drug ingredient or therapeutic moiety is absorbed from a drug product and becomes available at the site of drug action", which is also the definition used in the lawless wasteland of Wikipedia. A consensus statement produced by a group in Munich (Balant et al, 1989) changed the ending to  "becomes available at the site of drug action or in a biological medium believed to reflect accessibility to a site of action". This definition is more inclusive than the tablet-centric definitions from other textbooks because it takes into account the possibility that circulating drug levels may misrepresent its availability to its target. Consider phenytoin: as the only biologically active form of phenytoin is the free drug, the circulating protein-bound fraction (up to 90%) is not available to its site of action in the brain, even though it might be well-absorbed. 

Calculation of bioavailability

Bioavailability made ridiculously simple

Generally speaking, the bioavailability is calculated from the same dose being given IV and orally to some healthy volunteers who agree to have their blood regularly sampled for concentration testing.  In its purest form, oral bioavailability (F) can be described by the following equation:

simple bioavailability equation

This equation answers the most basic definition, as it describes the fraction of the moiety or whatever which ends up in the systemic circulation, as compared to some sort of maximum possible systemic concentration (assuming both the oral and IV doses of the drug are the same). However, the reader who is paying attention will soon realise that the concentration of a drug given IV will be maximal at Time Zero, whereas the concentration of an orally administered drug will be maximal at Time Whatever (depending on the rate of absorption). This calls for some sort of graph, and a mathematical relationship to describe it.

Dost's Law of Corresponding Areas

The calculation of bioavailability of drug doses which are getting absorbed at different rates rests on the premise that

"The ratio of the area beneath the blood level-time curves after oral administration to that following intravenous administration of the same dose is a measure of the absorption of the drug administered."

This is Dost's Law of Corresponding Areas, a mathematical concept used to determine the absorption of a drug from the area under the curve which is described by its serum concentration measurements over time. Those madmen lusting for robust German maths can avail themselves of Dost's original 1972 paper (though most of us never get farther than his zusammenfassung). For the rest of us, Pharmacokinetics Made Easy (Birkett, 2009) discusses the same concept but without actually calling it Dost's law (because, presumably, that would not be Made Easy). 

illustration of Dosts Law of Corresponding Areas

 The ability to describe Dost's Law or discuss it in any great detail has never been expected of CICM primary exam candidates and is unlikely to ever come up as a critically important part of a written paper.  In brief, the Law relates the area under the curve (AUC) of an IV dose to the AUC of an orally administered dose.  If the area under the oral concentration curve covers 50% of the area covered by the IV curve, the law dictates that the drug is 50% bioavailable. This mathematical relationship has been proven in a model-independent manner by the application of linear systems theory (Vaughan, 1977). For the casually interested user of (at most) primary school-level maths, this mathematical proof is of minimal relevance- it would suffice to agree with Vaughan that it is sound, and to keep using Dost's law. 

In summary, bioavailability should really be explained in terms of concentration/time curve AUCs.

Absolute bioavailability

The bioavailability of the intravenous dose of any drug is by definition 100%. Hence, the bioavailability of all other formulations and routes of administration can be compared to this reference value as an absolute standard, and from this we derive the equation for absolute bioavailability:

bioavailability equation for equal oral and IV doses

where F is the bioavailability fraction, and the AUC values are the area-under-the-curve measurements collected after IV and oral doses. 

Relative bioavailability

Whereas absolute bioavailability compares the drug formulation to an equivalent IV dose, relative bioavailability compares it to another similar non-IV formulation. One may be comparing two tablets, or a tablet to a suppository, or something similar. Welsh (2011) produces an excellent example of how one might devise a drug trial to test the relative bioavailability of two competing formulations.

Definition of bioequivalence

This has never been asked about in the exam, but could come up in some sort of viva scenario.  Birkett (2009) describes bioequivalence as "a clinical definition referring to two formulations of a drug"

"Drugs are considered bioequivalent if the extents and rates of absorption of drug from them are so similar that there is likely no clinically important difference between their effects"

Again, there are a variety of definitions with none clearly winning the compromise between brevity and accuracy. Most of them will include the words "rate", "extent", "absorption" and usually at some stage the similarity of clinical effect will be mentioned. 

Bioequivalence rests on the assumption that the measured drug concentration is related to its clinical effect. If the active ingredient from both drug formulations achieves the same concentration at the same rate (or AUC of the concentration over time curve) then likely these two drugs will achieve the same clinical effect, and so they can be considered bioequivalent. The definition leaves some interpretive wiggle-room, as the precise description of which clinical effect is relevant might vary for any given drug. Two drug formulations might be bioequivalent in terms of their therapeutic effect but differ markedly in their adverse effects (for example, PR diclofenac and oral diclofenac).

If one is able to agree on what one views as a clinically relevant effect, one may be able to test bioequivalence empirically. Typically one takes the serum concentration as the easily measurable and clinically relevant parameter. The two drugs are then compared in terms of their relative bioavailability. If the AUC ratio of the two drugs is within the range of 0.8-1.25, the drug company can claim that the two formulations are bioequivalent - this is the acceptable interval of bioequivalence (Rescigno, 1992). This is an important step in marketing a generic competitor product.

Factors which influence bioavailability

Question 5(p.2) from the first paper of 2010 specifically asked for factors which influence oral bioavailability. However, because drugs end up getting absorbed from lots of different sources, it is important to discuss non-oral bioavailability as well, considering especially that all factors were questioned in Question 22 from the second paper of 2017, where the college complained about how "nearly all candidates failed to provide a comprehensive list of factors affecting bioavailability";  many of the trainees limited themselves to factors affecting oral administration, as if they were still answering the question from 2010. . The pass rate was 33%.


Factors which Influence Drug Bioavailability

Generic influences on drug bioavailability

  • Drug concentration at site of administration
  • Surface area of the absorptive site
  • Drug pKa
  • Drug molecule size
  • pH of the surrounding fluid

Factors affecting first pass metabolism

  • Drug absorption from the gut
  • Drug metabolism in the gut
  • Metabolism in the gut wall
  • Metabolism in bloodstream (eg. plasma esterases)
  • Hepatic blood flow
  • Hepatic enzyme activity
  • Spontaneous drug degradation

Factors affecting gastrointestinal absorption

  • Gastric motility
  • Intestinal motility
  • Splanchnic perfusion
  • Tablet disintegration
  • Mode of transport
  • Intestinal content
  • Bile and bile salt content
  • Enterohepatic recirculation
  • Metabolism by gut bacteria
  • Metabolism in the intestinal wall
  • Drug on drug interactions in the gut
  • First pass metabolism

Bioavailability via transdermal and mucosal routes of administration

  • Mucosal blood flow
  • Drug lipophilicity
  • Factors affecting membrane penetration, eg. molecule size, pKa, etc
  • pH of the mucosal fluid

The college answer to Question 5(p.2) also mentions that a discussion of extraction ratio  (possibly with some equations) would have been helpful, and this undergoes some detailed exploration in the chapter on first pass metabolism and hepatic clearance.

The effect of shock on bioavailability

The table above has been converted to include the effects of shock on the listed factors:

Factor Effects of shock

Factors affecting gastrointestinal absorption

  • Gastric motility
Gastric motility and gastric emptying is decreased; the latter has the effect of decreasing absorption rate. The importance of gastric drug absorption becomes greater.
  • Intestinal motility
Intestinal motility is decreased, which slows gut transit. On one hand this has the effect of decreasing the rate of drug absorption because the delivery of the drug to absorptive surfaces is slowed. On the other hand, the increased duration of exposure to gut surfaces may increase the overall absorption of orally administered drugs, particularly in the context of overdose with a large bezoar of sustained release formulation tablets.
  • Splanchnic perfusion
Decreased splanchnic perfusion is a part of stereotypic shock response (and we make it worse by giving the patients noradrenaline). The ultimate effect is decreased drug transport from the gut wall to the systemic circulation. 
  • Metabolism by gut bacteria
Gut bacteria may metabolise more drugs if there is intestinal stasis and they have plenty of time to work on the drug in the dilated paralysed bowel loops. Alternatively, drug metabolism by bacteria may be completely abolished by the wholesale slaughter of these bacteria in the wake of high dose broad spectrum antibiotics. Colonic transit may also be increased by gut ischaemia and diarrhoea, resulting in less exposure time. 
  • Metabolism in the intestinal wall
An ischaemic intestine will not be metabolising anything. A poorly perfused shocked intestine will also be likely to downregulate its brush border enzymes, focusing on survival and self-preservation.
  • Intestinal surface area
This will be diminished in the wake of ischaemia as the villi are shed and the brush border denuded. A decrease in the intestinal surface area will be the result. 

Factors affecting first pass metabolism

  • Hepatic blood flow
This is sacrificed in shock, and drug metabolism will be slowed proportionally (particularly where the enzymes are not particularly saturable and blood flow determines the rate of metabolism)
  • Hepatic enzyme activity
The activity of hepatic enzymes may be downregulated (as in the case of CYP enzymes during septic shock) or abolished completely (as in the case of ischaemic hepatitis)
  • Shunts
In shock states with poor cardiac output and hepatic congestion (eg. cardiogenic shock) portosystemic shunts may open, allowing drugs to bypass first pass metabolism

Factors influencing absorption from other sites

  • Decreased mucosal perfusion
Buccal, rectal, vaginal absorption - these will be diminished or erratic because blood flow to these regions is usually sacrificed
  • Altered muscle and subcutaneous blood flow
Typically shock states decrease cutaneous and muscle blood flow, leading to mottling. This will result in decreased bioavailability of intramuscular and subcutaneously administered drugs. The only exception to this rule is anaphylaxis, where systemic vasodilation leads to excellent intramuscular absorption.
  • Tachypnoea
Increased respiratory rate and higher tidal volumes may improve the bioavailability of nebulised drugs and gaseous agents, or - instead - decrease it, if the patient is taking shallow peri-arrest breaths

Factors affecting the bioavailability of already absorbed drugs

  • Decreased protein binding
Most of the proteins which are expected to bind drugs will have their production downregulated during the acute phase response (eg. the hypoalbuminaemia of acute illness). Drug bioavailability will be increased by this if the drug is highly protein-bound.
  • Decreased plasma metabolism
The synthesis of plasma esterases and proteases will be decreased during an acute phase response, leading to diminished drug clearance by these enzymes.


Wesch, Roland. "Absolute and relative bioavailability." Drug Discovery and Evaluation: Methods in Clinical Pharmacology. Springer Berlin Heidelberg, 2011. 173-180.

Vaughan, D. P. "A model-independent proof of Dost's law of corresponding areas." Journal of pharmacokinetics and biopharmaceutics 5.3 (1977): 271-276.

Branson, Herman. "The kinetics of reactions in biological systems." Archives of biochemistry and biophysics 36.1 (1952): 48-59.

Dost, F. H. "Absorption, Transit, Occupancy und Availments als neue Begriffe in der Biopharmazeutik." Journal of Molecular Medicine 50.8 (1972): 410-412.

Balant, L. P. "Is there a need for more precise definitions of bioavailability?." European Journal of Clinical Pharmacology 40.2 (1991): 123-126.

Rescigno, Aldo. "Bioequivalence." Pharmaceutical research 9.7 (1992): 925-928.

Koch-Weser, Jan. "Bioavailability of drugs." New England Journal of Medicine 291.10 (1974): 503-506.

Allam, Ahmed N., S. S. El Gamal, and V. F. Naggar. "Bioavailability: A pharmaceutical review." Int J Novel Drug Deliv Tech 1.1 (2011): 77-93.

Pond, Susan M., and Thomas N. Tozer. "First-pass elimination basic concepts and clinical consequences." Clinical pharmacokinetics 9.1 (1984): 1-25.

De Paepe, Peter, Frans M. et al  "Pharmacokinetic and pharmacodynamic considerations when treating patients with sepsis and septic shock."  Clinical pharmacokinetics 41.14 (2002): 1135-1151.

Dost's law is also discussed with a medium level of mathematical wizardry in the 4th chapter of Atkinson's Principles of Clinical Pharmacology - and we have Jade to thank for this find