Choosing an assay kit by what it can actually be asked: why a comet assay kit measures strand breaks per cell and is scored rather than read, what cell proliferation assays and a cell proliferation assay kit report about metabolic activity rather than cell number, where a mycoplasma test kit belongs on a schedule and not in a crisis, what an endotoxin detection kit, an endotoxin assay, an endotoxin assay kit and an endotoxin test kit have to demonstrate about inhibition and enhancement before a result is valid, how a pyruvate assay kit, a protein assay kit and a cytokine assay kit each depend on a standard you have to choose, what a complement c3 elisa measures that a functional assay does not, where immunohistochemistry reagents and an immunohistochemistry kit are a system rather than a reagent, what a luminescence reader and a luminometer plate contribute to any of them, and what red fluorescent protein and aav quantification kits are for

Choosing and validating a kit

1. Match the endpoint to the question. Metabolic activity, membrane integrity, cell number and strand breaks are different endpoints that behave differently. Choosing by convenience produces a number that answers a question nobody asked.
2. Check the matrix the kit was validated in. A kit validated in buffer or serum may not work in lysate, medium or a process intermediate. Ask what matrix the validation used and run a spike recovery in yours.
3. Test for interference explicitly. Coloured compounds, high protein, chelators and inhibitors all interfere. A compound only control without cells, and a spiked sample, detect most of it in one plate.
4. Understand the standard. Every quantitative kit reports relative to its standard, and standards differ between suppliers. Switching kit changes the numbers, and comparing across kits needs a bridging experiment.
5. Run scored assays blind and to a rule. Assays scored by eye or by image analysis depend heavily on the rule and the scorer. Fix the rule, score blind, and report both.
6. Keep schedule assays on a schedule. Contamination screening is only useful before the results depend on it. Testing when something looks wrong means the affected experiments are already done.

Kits carry assumptions

A kit encodes a validation that someone else did, in a matrix that may not be yours, against a standard you did not choose. Using it inside those assumptions is efficient; using it outside them silently is where results go wrong.

Read the insert for the matrix and the standard before the protocol. Those two fields decide whether the kit answers your question.

Interference is checked in one plate

A compound only control, a matrix blank and a spiked recovery together detect almost every interference that matters, and they fit on one plate alongside the first real experiment.

Doing it once per new sample type converts a recurring source of confusion into a known property.

Common questions

Why do two kits give different numbers?

Different standards and different endpoints. Both can be internally correct, which is why a study should fix one kit and state it rather than mixing them.

What does an endotoxin assay need before a sample?

An inhibition and enhancement test in your matrix, showing that a spiked known amount is recovered. Without it a low result cannot be distinguished from a suppressed assay.

Is a proliferation kit measuring cells?

Usually it measures metabolic activity, which tracks cell number only when metabolism per cell is constant. Compounds that affect metabolism directly break that assumption.

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