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Short answer
alamarBlue is Thermo Fisher Scientific’s trade name for a ready-to-use, resazurin-based reagent that works as a cell health indicator. Living cells reduce resazurin to resorufin, which is red and highly fluorescent, so the alamar blue assay (generically, a resazurin assay) reports metabolic activity that is proportional to the number of living cells. It is read on a plate reader by fluorescence or absorbance.
Definition and scope
Thermo Fisher describes alamarBlue as a non-toxic, resazurin-based solution that functions as a cell health indicator to quantitatively measure viability, and as a viability and proliferation indicator through the conversion of resazurin to resorufin (product page, product information sheet, read 2026-11-07). The product page says it is applicable to a wide variety of mammalian cells, bacteria, plants and fungi.
alamarBlue versus “resazurin assay”
alamarBlue is a brand; “resazurin assay” is the generic description of the chemistry. The page you search for as “alamar blue assay” and the one you search for as “resazurin assay” describe the same readout: conversion of resazurin to resorufin by living cells. The ATP assay guide lists PrestoBlue alongside alamarBlue in the same resazurin category.
Standard and HS versions
Thermo Fisher’s product page lists two sensitivities: it states that alamarBlue HS can detect as few as 20 cells per well and that standard alamarBlue can detect as few as 50 cells per well.
How it is used in practice
The manufacturer’s microplate protocol (Thermo Fisher, read 2026-11-07) gives the following parameters. Check them against your own instrument and plate before relying on them.
| Parameter | Manufacturer’s stated value |
|---|---|
| What changes | Resazurin is reduced to resorufin inside living cells; resorufin is red and highly fluorescent |
| Fluorescence read | Excitation/emission 560/590 nm |
| Absorbance read | 570 nm, with 600 nm as the reference wavelength |
| Incubation | 37 °C for 1-4 hours |
| Signal stability | Stable for 7 hours |
| Background control | Wells containing only cell culture medium (no cells) on each plate |
| Read orientation | Bottom-read is more sensitive than top-read |
Because the reagent enters live cells, the manufacturer states there is no need to lyse or further process cells, and that cells continue to proliferate normally if the reagent is replaced with complete medium after reading. The product page also says the dye is stable in culture media including media containing phenol red, and that other functional assays can be performed after the viability measurement. Instrument filters and read modes are covered in the plate reader guide.
Related concepts: what each viability readout measures
Choosing a readout is a question of what the signal reports. The comparison below uses only what the manufacturer and the site’s sibling guides state.
| Readout | What the signal reports | Note |
|---|---|---|
| alamarBlue / resazurin | Metabolic reduction of the dye by living cells; signal proportional to living cell number | Described by Thermo Fisher as non-toxic; no lysis required |
| MTT | Colorimetric tetrazolium reduction | Forms an insoluble purple formazan product |
For ATP-based readouts see the ATP assay guide. For how resazurin sits among DNA-synthesis, dye-dilution and other metabolic readouts see cell proliferation assays compared, and for a related tetrazolium-type method see the CCK-8 assay guide.
Where it matters for lab decisions
Three points in the manufacturer’s protocol affect how results are generated and compared between plates and labs:
- Include no-cell medium wells on every plate so background fluorescence can be corrected.
- Record the incubation time and read mode (top or bottom): the protocol gives a 1-4 hour window and states bottom reads are more sensitive.
- Read within the stated 7-hour stability window.
Because the reagent is non-lytic, the manufacturer notes the same wells can go on to other assays.
Common misunderstandings
- “alamar blue” and “resazurin assay” are different tests. They describe the same resazurin-to-resorufin readout; alamarBlue is one brand of it.
- It counts cells directly. The signal reports metabolic activity of living cells, which the manufacturer states is proportional to cell number; it is a proxy, not a cell count.
- It only works in fluorescence. The manufacturer gives both fluorescence (560/590 nm) and absorbance (570 nm, 600 nm reference) read options.
FAQ
What is alamarBlue?
A ready-to-use, resazurin-based reagent from Thermo Fisher Scientific that functions as a cell health indicator.
Is alamarBlue the same as a resazurin assay?
The chemistry is the same: living cells reduce resazurin to resorufin. alamarBlue is a brand name; “resazurin assay” is the generic term.
Does alamarBlue kill cells?
Thermo Fisher describes it as non-toxic, permitting long-term exposure of cells without negative impact, with no lysis needed.
Fluorescence or absorbance?
Either. Fluorescence is read at 560/590 nm excitation/emission; absorbance at 570 nm with a 600 nm reference.
How long is the incubation?
The microplate protocol states 1-4 hours at 37 °C, with the signal stable for 7 hours.
Sources
- Thermo Fisher Scientific, alamarBlue Cell Viability Reagent for microplates protocol, read 2026-11-07.
- Thermo Fisher Scientific, alamarBlue rapid and accurate cell health indicator, read 2026-11-07.
- Thermo Fisher Scientific, alamarBlue product information sheet (PDF), read 2026-11-07.
