# Arrhenius vs ASAP vs AccelStab: which stability model for excursion review?

> The shared Arrhenius foundation, what ASAP (humidity) and AccelStab (non-linear) add, a side-by-side table, and which fits a development shelf-life prediction versus an operational excursion release decision.

- Type: Article · Jul 2026 · 6 pages · 6 min read
- Author: Adam Karpiński, Synlogica
- Canonical: https://synlogica.ai/resources/arrhenius-vs-asap-vs-accelstab/

## 1. The short answer

Classic **Arrhenius/MKT**, ** ASAP** and ** AccelStab** are often discussed as rivals. They are not — they answer different questions. Three of them predict a *development* shelf life from short, stressed studies; the operational job of an **excursion review** is different: "given this product's already-characterised kinetics and this actual logger trace, is this batch releasable, and with what confidence?" That is a probabilistic Arrhenius question, not a predictive-modelling one.

**Bottom line** Pick by the question, not by which model is newest. Shelf-life prediction from stressed studies → ASAP or AccelStab. Release decision after an excursion → probabilistic Arrhenius over the actual profile, with a stated confidence interval.

## 2. The shared foundation

All three inherit the Arrhenius temperature dependence of the degradation rate constant:

**k(T)** = A · exp(−** Eₐ** / (R · T))

**A** = pre-exponential factor · ** Eₐ** = activation energy · ** R** = 8.314 J/(mol·K) · ** T** = kelvin

What each method adds on top of this base — a humidity term, a non-linear degradation form, an uncertainty model — is exactly what distinguishes them.

## 3. The three approaches

### Classic Arrhenius & MKT

The baseline: rate depends on temperature via Arrhenius; MKT collapses a varying profile into one equivalent temperature. Simple, well-understood, sufficient when temperature is the dominant stress. Its blind spots are humidity and non-linear degradation.

### ASAP — humidity-corrected Arrhenius

The Accelerated Stability Assessment Program adds a **relative-humidity term** to the Arrhenius rate, so it models moisture-driven degradation. Its strength is fast, predictive shelf-life estimates from short high-stress studies — a development tool for products where humidity matters.

### AccelStab — Šesták–Berggren kinetics

An open-source R package fitting the **Šesták–Berggren** kinetic model, which describes ** non-linear degradation curves** (autocatalytic, sigmoidal) that a straight Arrhenius line misses. Also a predictive/development tool, stronger where the degradation shape is complex.

## 4. Side by side

| Aspect | Classic Arrhenius/MKT | ASAP | AccelStab | |

| Models humidity | No | Yes | Not primarily | |

| Degradation shape | Linear (log) | Linear (log) | Non-linear | |

| Primary job | Baseline / MKT | Predict shelf life | Predict shelf life | |

| Best when | Temperature dominates | Humidity-sensitive | Complex kinetics | |

| Uncertainty stated | Optional | Yes (predictive) | Yes (fit CI) | |

## 5. Which one, when

For **development shelf-life prediction** from short stressed studies: ASAP if humidity drives the failure, AccelStab if the degradation curve is non-linear, classic Arrhenius if temperature dominates and simplicity is fine.

For an **operational release decision after an excursion**, none of the predictive tools is the right frame. The product's kinetics are already characterised; the question is how much of the stability budget this specific event consumed, with a defensible confidence interval. That is what [Synlogica Terminus](https://synlogica.ai/) Quality (Terminus M4) computes — a probabilistic Arrhenius model over the actual logger trace with Monte-Carlo uncertainty propagation, packaged as a sealed decision. The full four-way comparison, with equations and assumptions, is in the [stability models compared white paper](https://synlogica.ai/resources/stability-models-compared/).

## 6. FAQ

### Which stability model should I use for excursion review?

For an operational release decision after an excursion, a probabilistic Arrhenius model over the actual profile (with stated uncertainty) fits best — it answers "is this batch releasable and with what confidence". ASAP and AccelStab are development tools that predict shelf life from short stressed studies, a different question.

### What is the difference between Arrhenius, ASAP and AccelStab?

All three build on Arrhenius temperature dependence. Classic Arrhenius/MKT is the baseline; ASAP adds a humidity term; AccelStab fits non-linear (Šesták–Berggren) curves. They differ in what they model on top of the shared kinetics and whether the job is prediction or release.

### Is ASAP better than classic Arrhenius?

Not better, different. ASAP is stronger when humidity drives degradation and you want a fast predictive shelf life from a short study. Classic Arrhenius/MKT is simpler and sufficient when temperature dominates. Pick by the failure mode and the question, not by which is newest.

## 7. References

- Waterman K.C. et al. — The Application of ASAP to QbD for Drug Product Stability. AAPS PharmSciTech.

- Humidity-corrected Arrhenius equation — Int. J. Pharmaceutics (2016).

- AccelStab — Accelerated Stability Kinetic Modelling (CRAN / github.com/AccelStab/AccelStab).

- ICH Q1A(R2); ICH Q1E — Stability Testing and Evaluation.