CI(lin) Calculator
CIlin Calculator – Linear Combination Index for Drug Synergy Analysis
The CIlin Calculator is an Excel-based scientific tool for the evaluation of drug synergy, additivity, or antagonism using the Combination Index (CI) approach derived from the Loewe additivity model.
This method is specifically developed for linear or non-normalizable dose–response data, where classical synergy models cannot be reliably applied.
The CIlin Excel calculator file is available for download here (XLSX file, ca 500 kB).
What problem does CIlin solve?
Most drug interaction models require normalized dose–response curves, typically sigmoidal for Hill-type models or hyperbolic for Michaelis-Menten-type systems.
However, in many experimental systems:
- the dose–effect relationship appears linear within the measurable range
- the maximal effect is unknown
- data cannot be normalized to a 0–1 scale
- technical limitations prevent observation of the response plateau
In such cases, standard drug synergy analysis methods, such as Bliss independence or the classical Loewe Combination Index, become unreliable.
The CIlin method overcomes this limitations by:
- working directly with experimentally measured effect values
- using linear regression to describe dose–effect relationships
- providing a Combination Index applicable to linear dose–effect data
What does the calculator provide?
The CIlin calculator computes:
• a single Combination Index (CIlin) describing the interaction across the studied dose–effect region
• classification of interaction:
- CI < 1 → synergistic interaction
- CI = 1 → additive effect
- CI > 1 → antagonistic interaction
This makes CIlin a practical drug synergy calculator for experiments with linear dose–response relationships.
Suitable for
- pharmacology
- pharmacognosy
- molecular biology
- biochemistry
- biomedical and life sciences research
- any system with apparently linear dose–effect relationships
Documentation
A PDF user manual explaining data input and result interpretation for the CIlin Calculator is available here (PDF file, ca 600 kB).
Academic use
The CIlin Calculator is freely available for academic and research purposes.
If the calculator is used in scientific work, please cite the original publication describing the CIlin method.
How to cite CIlin
If you use the CIlin Calculator in your research, please cite the original article describing the method:
Novotný B., Nagy M., Kurin E.
Synergy evaluation of non-normalizable dose-response data: Generalization of combination index for the linear effect of drugs.
Pharmaceutical Statistics. 2021;20(6):982-989. https://doi.org/10.1002/pst.2118
Recommended citation statement for Methods sections
Drug interaction analysis was conducted using the CIlin (Linear Combination Index) method, an extension of the Loewe additivity model developed for non-normalizable or apparently linear dose–effect relationships, where classical normalization-based synergy models are not applicable. Calculations were performed using an Excel-based CIlin calculator implementing this approach (Novotný et al., 2021).