Controls make an experimental comparison interpretable by addressing particular alternative explanations or showing that a measurement can respond. Positive, negative and vehicle controls have different roles, although one condition may serve more than one role in a specific assay. The useful question is what each control tests.
Identify what each control is intended to show
A positive control is expected to produce a defined response under the assay conditions. A negative control provides a comparison lacking the factor expected to produce that response. A vehicle control matches the solvent or carrier exposure without the test compound.
The Assay Guidance Manual’s cytotoxicity chapter illustrates separate background, vehicle-treated cell and positive-control conditions. Their distinct roles show why one generic control label may be insufficient.Riss and colleagues — Cytotoxicity Assays: In Vitro Methods to Measure Dead Cells (opens in a new tab)
| Control | Question it can address |
|---|---|
| Positive | Can this system detect an expected response? |
| Negative | What occurs without the response-producing factor? |
| Vehicle | What occurs with the carrier but no test compound? |
| Background | What signal comes from the measurement context alone? |
The categories depend on the method. A vehicle condition can also be the negative control, while a no-cell background answers a different question from living cells exposed to vehicle.
Distinguish no treatment from carrier exposure
In an original hypothetical cell experiment, the test material is delivered in a carrier. Comparing it only with completely untreated cells leaves the material and carrier effects combined in the contrast.
A matched vehicle condition helps isolate the difference associated with adding the test material to that carrier context. It still does not establish that the carrier is inert under every possible condition.
Check whether the vehicle exposure is actually comparable across the relevant groups. A condition receiving a different carrier amount does not answer exactly the same comparison, even when the methods call it the vehicle control.
Interpret a working positive control narrowly
Suppose the positive control produces the expected signal while the peptide condition does not. That provides evidence that the assay could show that control response in the run. It does not by itself establish that the peptide has no biological effect in every setting.
The peptide may be evaluated at a different target, exposure context or response pathway from the control. Whether the negative test result is informative depends on the assay’s suitability for the actual question.
Conversely, a peptide response resembling the positive control does not prove a shared mechanism. Different processes can produce the same downstream measurement, so mechanistic attribution requires additional discriminating evidence.
If the positive control fails, examine the study’s predefined run-validity criteria and investigation. A test result should not simply be declared interpretable because it looks interesting despite an unresolved control failure.
Read controls alongside the result and analysis
Inspect the control values, variability and any exclusions rather than relying only on a sentence saying appropriate controls were used. The data should show how the control conditions relate to the displayed comparison.
Normalising a control to 100% can make a plot easier to read but can hide its absolute signal and run-to-run variation. Check the underlying values when those features matter to the interpretation.
In the fictional carrier example, write the conclusion as a comparison with the matched vehicle under the tested conditions. Replacing that with better than no treatment may describe a comparison that was never made.
Controls also remain specific to an experiment. A positive control in one figure does not automatically validate a different assay elsewhere in the paper. Follow the connection between control, measured endpoint and conclusion for each experiment.
Sources and further detail
- Riss and colleagues — Cytotoxicity Assays: In Vitro Methods to Measure Dead Cells (opens in a new tab)
Assay Guidance Manual, published 1 May 2019; control descriptions for DNA-binding-dye and LDH experiments read. Used only for control roles; no reagent quantities, exposure recipe or assay protocol reproduced. Hypothetical comparisons are original.
Sources checked 19 September 2026. Worked examples are illustrative unless a supplied report is explicitly identified. This article has not undergone independent scientific peer review.