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Dictionary of Evidence-based Medicine 15 Bioavailability Bioavailability is a term used to describe the absorption characteristics (rate and extent) of a drug from a formulation. The indices used for comparative purposes are the extent of absorption, usually assessed by area under the blood level-time curve (AUC), the maximum concentration achieved in the blood (C ) and the time (T ) taken to reach theC ma , The term 'absolute bioavailability' is used when the bioavailability of a drug from a formulation is compared against that of a rapid intravenous equivalent dose. When the comparator is another dosage form administered by a route other than intravenous, the term 'relative bioavailability' is used. For drugs not intended to be absorbed into the bloodstream, bioavailability may be assessed by measurements intended to reflect the rate and extent to which the active ingredient or active moiety becomes available at the site of action. Bioequivalence studies Bioequivalence studies are undertaken to compare the bioavailability of a drug from two formulations, usually with a view to showing that the two formulations are bioequivalent (i.e. have the same bioavailability). For systemic drugs, many drug regulatory authorities consider two formulations as being bioequivalent if the 90% confidence interval for the ratio of the area under the blood level-time curve (AUC) is in the range 0.80-1.25. Tighter ranges may be required for drugs with particularly narrow therapeutic ranges. The C max ratio (see Bioavailability) is allowed to vary more widely than the AUC ratio provided this can be clinically justified. For assessing equivalence of T max (see Bioavailability), the test statistic is the difference in the values for the test and reference product (Chow SC, Lui JP (1992) Design and analysis of bioavailability and bioequivalence studies. Marcel Dekker, New York. Midha KK, Blume HH (eds) (1993) Bioavailability, bioequivalence and pharmacokinetics. Medpharm, Stuttgart). Regulatory authorities such as the US Food and Drug Administration (FDA) accept that bioequivalence may sometimes be demonstrated using in vitro bioequivalence standard, especially when such an in vitro test has been correlated with human in vivo bioavailability data. In other situations, bioequivalence may sometimes be demonstrated through comparative clinical trials or pharmacodynamic studies.
16 Dictionary of Evidence-based Medicine Bioequivalent drug products The US Food and Drug Administration (FDA) uses this term to describe pharmaceutically equivalent or alternative products that display comparable bioavailability when studied under similar experimental conditions. In particular, (a) the rate and extent of absorption of the test drug do not show a significant difference from those of the reference product when administered at the same molar dose of the therapeutic ingredient under similar experimental conditions in either a single dose or multiple doses; or (b) the extent of absorption of the test product does not show a significant difference from the extent of absorption of the reference product when administered at the same molar dose of the therapeutic ingredient under similar experimental conditions in either a single dose or multiple doses and the difference from the reference product in the rate of absorption is intentional, is reflected in its proposed labelling, is not essential to the attainment of effective body drug concentrations on chronic use and is considered medically insignificant for the drug. When the above are not applicable (e.g. for drug products that are not intended to be absorbed into the bloodstream), other in vivo or in vitro tests of bioequivalence may be appropriate. Biological efficacy test (see under Intention to treat analysis) Biometrics Biometrics refers to statistical sciences applied to the study of biological phenomena. The term can be used synonymously with biostatistics. Biostatistics Biostatistics refers to statistical sciences applied to the study of biological phenomena. The term is used synonymously with biometrics and increasingly with medical statistics. Blocking In the design of experiments, blocking is an approach which can be used to eliminate the effect of a factor which is known to influence the result but whose effect is not of direct interest. For example, if an experiment cannot be completed in one day and day-to-day variation is known to affect the result, the experiment can be divided using day as a block factor. The block or day effect can then be eliminated before computation of the model. This
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Page 4 and 5: Preface This book was conceived at
Page 6 and 7: A Ability to pay Ability to pay is
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Page 14 and 15: B Balanced design In an experiment,
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G Game theory Game theory, first de
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I ICD (see International Classifica
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K Keynes, John Maynard (1883-1946)
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M Macroeconomics Macroeconomics is
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N N of I trial An N of 1 trial is a
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o OBM (see Opinion-based medicine)
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Figure 25 Measuring quality of life
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S Scarcity Scarcity is often referr
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u Unique value effect This term is
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Z Z variate A random variable which
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