larger dynamic range observed with the alamarBlue HS compared to the standard alamarBlue formulation

alamarBlue HS and alamarBlue Cell Viability Reagents are ready-to-use, non-toxic, resazurin-based solutions that function as cell health indicators to quantitatively measure viability. alamarBlue uses the natural reducing power of living cells to convert resazurin to fluorescent resorufin. alamarBlue HS (High Sensitivity) Cell Viability Reagent is a superior version of alamarBlue that retains all the key alamarBlue reagent characteristics, but also provides much higher sensitivity.

alamarBlue HS and alamarBlue are recommended in cases of extended viability studies or when using a high cell density.

Selection guide

 alamarBlue HS (high sensitivity) Cell Viability ReagentalamarBlue Cell Viability Reagent
  • Microplate assay recommended in cases of extended viability studies or when using a high cell density
  • Applicable to a wide variety of mammalian cells, bacteria, plants, and fungi
Mechanism of detection
  • Resazurin is converted to fluorescent resorufin
Detect as few as 20 cells per well

Detect as few as 50 cells per well
Reagent purity
  • Purified
  • Varying concentrations of resorufin contamination depending on sources of material and manufacturing conditions
Background fluorescence
  • Low
  • >50% reduction in background fluorescence
  • Moderate
  • Increased background fluorescence due to presence of resorufin contamination
  • High
  • >100% ratio increase
  • Moderate
  • Reduced signal-to-background due to presence of resorufin contamination
Assay signal window
  • High
  • >10-fold increase as compared to the non-HS assay
  • Moderate
  • Reduced dynamic range due to the presence of resorufin contamination
Cat. No.A50100DAL1025

alamarBlue HS reagent is recommended in cases of extended viability studies or when using a high cell density, whereas PrestoBlue HS reagent is recommended for quick viability determination (10-minute incubation).

Learn more about PrestoBlue and PrestoBlue HS

What are alamarBlue assays?

Highly referenced for cytotoxicity and viability assays, alamarBlue cell viability assay has been used for years in biological and environmental studies (1). Furthermore, analysis of cell proliferation and cytotoxicity is a vital step in evaluating cellular health and in the drug discovery process. alamarBlue is a proven cell viability indicator that uses the natural reducing power of live cells to convert resazurin to the fluorescent molecule, resorufin. Analysis can be evaluated quantitatively on an absorbance- or fluorescence-based microplate reader; while qualitative analysis can be evaluated by the visual color change of the solution which is indicative of metabolically active cells.

The active ingredient of alamarBlue (resazurin) is a non-toxic (Figure 1), cell permeable compound that is blue in color and virtually non-fluorescent. Upon entering cells, resazurin is reduced to resorufin, which produces a very bright red fluorescence (Figure 2). Viable cells continuously convert resazurin to resorufin, thereby generating a quantitative measure of viability—and cytotoxicity.

Figure 1. Demonstrated Non-Toxicity of the alamarBlue HS Cell Viability Reagent. U2OS Cells were treated with and without alamarBlue HS for 4 hours. CyQUANT Direct reagent was added to both populations following manufacturer’s instructions. Based on the CyQUANT Direct fluorescence measurements there were no significant differences in fluorescence measurements suggesting alamarBlue HS does not significantly contribute to cellular toxicity. Additionally, when exposed to varying concentrations of gambogic acid, the U2OS cells pre-treated with alamarBlue HS displayed similar IC50 values as the control cells.

Chemical structures of resazurin and resorufin

Figure 2. Metabolically active cells convert resazurin to resorufin, a red-fluorescent indicator. Resazurin is a non-fluorescent indicator dye that undergoes chemical reduction to bright red-fluorescent resorufin in metabolically active cells. The amount of fluorescence produced is proportional to the number of living cells. In alamarBlue HS, the resazurin is purified and provides superior performance compared to the standard alamarBlue reagent which has known contamination issues that lead to sub-optimal performance.

alamarBlue has a proven track record as an indicator of cell health and its nontoxic nature permits long-term exposure of cells without negative impact; cells grown in the presence of alamarBlue were found to produce similar numbers of viable cells as control cells, as determined by flow cytometric analysis of CD44, CD45RB, and CD4 antigens.

The alamarBlue HS reagent refers to the fact that the alamarBlue HS reagent contains purified resazurin, devoid of any contaminating resorufin which can negatively impact reagent performance. The alamarBlue HS reagent provides a >50% reduction in background signal and >100% signal-to-background ratio increase compared to the standard alamarBlue reagent, while retaining all the key characteristics that make alamarBlue a highly-referenced cell viability and cytotoxicity reagent.

How do I use alamarBlue assays?

alamarBlue HS and alamarBlue cell viability reagents are easy add-and-read assays (Figure 3). Simply add alamarBlue HS or alamarBlue to your cells, incubate for 1–4 hours, and read the fluorescence or absorbance. The amount of fluorescence or absorbance is proportional to the number of living cells and corresponds to the cell’s metabolic activity. Damaged and non-viable cells have lower innate metabolic activity and thus generate a proportionally lower signal than healthy cells.

alamarBlue assays are compatible with multiple instrument platforms. After incubation with alamarBlue HS or alamarBlue, your samples can readily be measured on fluorescence and absorbance instrumentation. For fluorescence, simply set up your plate reader or fluorescence spectrophotometer using 560/590 nm (excitation/emission) filter settings. Alternatively, the absorbance of alamarBlue HS and alamarBlue can be read on a UV-Vis spectrophotometer at 570 nm.

Finally, analyze results by plotting fluorescence intensity (or absorbance) versus compound concentration. While results are linear and quantitative for both fluorescence and absorbance, fluorescence readings provide higher sensitivity.

alamarBlue HS workflow - reagent product, microplate, incubate time, microplate reading, data graph

Figure 3. How alamarBlue assays work.

alamarBlue assays do not require cell lysis

alamarBlue HS easily enters into live cells, eliminating the need to lyse or further process cells using fixation and DNA denaturation techniques. The dye is stable in cell culture media, including media containing phenol red. These characteristics allow you to:

  • Continuously monitor the effects of drugs in a dose response assay or when optimizing drug concentrations
  • Perform other functional assays after cell viability measurements with alamarBlue HS or alamarBlue are complete

alamarBlue HS has improved performance

As a result of the synthesis and manufacturing processes, all resazurin-based reagents contain a detectable amount of highly fluorescent resorufin contamination. The amount of the resorufin contamination can vary greatly between sources of the material and manufacturing conditions. The varying amounts of resorufin contribute to differences in detectable background fluorescence. Additionally, the contaminating resorufin and the resulting higher background signal significantly reduces the signal-to-background ratio and dynamic range of the assay.

To improve the performance of the resazurin-based reagents, an innovative process was developed and, on implementation, removes the contaminating resorufin resulting in purified resazurin. This purified resazurin was used in the standard formulation creating the multiple component alamarBlue HS Cell Viability Reagent.

alamarBlue HS has higher sensitivity compared to alamarBlue

The purified resazurin used for alamarBlue HS reagent results in a >50% decrease in background fluorescence and a >100% increase in the signal-to-background ratio (Figures 4 and 5).

Figure 4. Significantly lower background fluorescence observed when comparing alamarBlue HS vs alamarBlue. The purified resazurin used for the alamarBlue HS formulation results in a cell viability reagent displaying a background fluorescence reduction in suspension (Ramos), primary cells (HCASMC), and adherent (U2OS) cells.

3 bar graphs showing alamarBlue HS signal-to-background  ~5-15x higher what is observed for alamarBlue in tested cell types

Figure 5. Performance improvement in signal to background ratio with alamarBlue HS as compared to alamarBlue. Improved cell viability signal to background ratios were displayed with HCASMC (primary cells), Ramos (suspension cells), and U2OS (adherent cells) when using the alamarBlue HS cell viability reagent which contains purified resazurin as compared to the standard alamarBlue formulation.

alamarBlue HS has a larger assay window compared to alamarBlue

The alamarBlue HS reagent displays an extended viability detection time window for a wide variety of organisms and thus can be used with various human and animal cell lines, bacteria, plant, and fungi, resulting in quantitative measurement of cell viability and proliferation (Figure 6).

alamarBlue HS signal-to-background normalized to 100, decreases to 0. alamarBlue is ~15 on the same scale decreasing to 0.

Figure 6. Much larger dynamic range observed with the alamarBlue HS Cell Viability Reagent which contains purified resazurin as compared to the standard alamarBlue formulation. Various concentrations of gambogic acid were added to A549 cells and the changes in viability were detected. The results demonstrate a significant improvement in the dynamic range of the assay when using alamarBlue HS.

Thermo Scientific plate readers

Expertly detect fluorescence with Thermo Scientific plate readers

High-sensitivity fluorescence detection for 96–1,536 samples can be quickly performed on Fluoroskan or Fluoroskan FL Microplate Fluorometer or Varioskan LUX Multimode Microplate Reader using Invitrogen reagents for optimal detection. Take advantage of automatic dynamic range selection to get optimal gain settings for each individual well and automation capabilities for even higher throughput.

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Cell Analysis Learning Center—Find educational resources such as application notes, webinars, videos, articles, and more that cover the use of many of our reagents and kits for cell analysis.

Fluorescence SpectraViewer—Online tool for visualization of the excitation and emission of fluorescent reagents; allows for checking spectral compatibility for multiple fluorophores.

Application notes

 Application note: Evaluation of the toxic effects of PFOS in hESCs differentiating into cardiomyocytes

 Application note: Monitoring cell health with alamarBlue and PrestoBlue reagents using the Varioskan LUX Multimode Microplate Reader

BioProbes article

An Absorbance-Based Assay for Cell Health and Proliferation


 New and improved cellular health evaluation of 2D and 3D cellular models using microplate reader assays


Cell Analysis Support Center—Find technical information, tips and tricks, and answers to everyday problems.