Published on 21/12/2025
Mastering the TOST Procedure in Bioequivalence Studies
Introduction: What is the TOST Procedure in BA/BE?
The Two One-Sided Tests (TOST) procedure is the gold standard statistical approach used in bioavailability and bioequivalence (BA/BE) studies to determine if two drug products are equivalent in terms of their pharmacokinetic profiles. Rather than testing for a difference, TOST tests for equivalence — an essential distinction in regulatory science. It evaluates whether the 90% confidence interval (CI) for the geometric mean ratio (GMR) of key pharmacokinetic parameters, such as Cmax and AUC, falls entirely within predefined bioequivalence limits (typically 80.00% to 125.00%).
Regulatory bodies including the European Medicines Agency (EMA), U.S. FDA, and WHO recommend TOST as a primary analysis tool in BE studies.
Key Concepts Underlying TOST
TOST operates on the principle that bioequivalence can only be claimed if the entire confidence interval lies within the equivalence margin. The standard hypotheses are set up as:
- Null Hypothesis (H0): The GMR is outside the bioequivalence range of 80.00% to 125.00%.
- Alternative Hypothesis (H1): The GMR is within the bioequivalence range.
This is assessed by performing two one-sided t-tests at the α level of 0.05, corresponding to the use
Step-by-Step Execution of the TOST Method
- Log-transform the pharmacokinetic data (e.g., ln(Cmax), ln(AUC)).
- Fit the ANOVA model including fixed effects for sequence, period, treatment, and subjects nested within sequence.
- Estimate the GMR (Test/Reference) from least square means.
- Construct the 90% confidence interval using the residual variance from the ANOVA.
- Back-transform the lower and upper CI bounds to the original scale.
- Compare the CI against the BE limits of 80.00% to 125.00%.
Illustrative Example
Let’s say a BE study comparing a generic vs innovator formulation yields a GMR for AUC of 0.94 and a 90% CI of 0.89–1.01. Since the entire CI lies within the 80.00%–125.00% range, the products are considered bioequivalent.
Dummy Table: TOST Evaluation Output
| Parameter | GMR | 90% CI | Bioequivalence Conclusion |
|---|---|---|---|
| Cmax | 0.92 | 0.88 – 0.96 | Yes |
| AUC0–t | 0.97 | 0.93 – 1.01 | Yes |
Assumptions and Limitations of TOST
For valid interpretation, TOST relies on several assumptions:
- Log-normal distribution of PK data
- Homogeneity of variance
- Normality of residuals
- Randomized treatment sequence
When these assumptions are violated, alternative methods like non-parametric tests or mixed-effects models may be considered.
Regulatory Expectations and Guidance
Agencies such as the U.S. FDA and EMA expect BE studies to use TOST with clearly stated hypotheses and transparent statistical methods. According to guidance:
- The CI must be calculated on log-transformed data.
- Analysis should be performed using validated statistical software.
- The method must be predefined in the Statistical Analysis Plan (SAP).
- Both AUC and Cmax must meet bioequivalence criteria independently.
Real-World Case Study: TOST in a Generic Antifungal Submission
In a pivotal BE study evaluating a generic fluconazole 150 mg tablet, the TOST approach yielded the following results:
- GMR for Cmax = 0.98; 90% CI: 0.91 – 1.06
- GMR for AUC = 1.01; 90% CI: 0.96 – 1.08
Both intervals were comfortably within the 80.00%–125.00% limits, and the ANDA was approved based on successful TOST-based demonstration of bioequivalence.
Alternative Approaches for Highly Variable Drugs
For highly variable drugs (HVDs), the widened acceptance criteria (scaled average bioequivalence) may apply. TOST is still the core method but is modified with scaling factors based on intra-subject variability. These adjustments must be justified using replicate study designs and variability thresholds.
Conclusion: TOST as a Cornerstone of BE Evaluation
The TOST procedure offers a robust, transparent, and widely accepted method to statistically demonstrate bioequivalence. By focusing on equivalence rather than difference, it ensures that generic drugs meet strict regulatory requirements for therapeutic equivalence. Proper application of TOST — backed by sound assumptions and clear documentation — is essential for successful BA/BE submissions.