Assay kits are research tools used in a broad range of applications, such as studying biological pathways, investigating diseases, drug discovery, process development, and environmental monitoring.
In contrast to other analytical methods, such as HPLC or mass spectrometry, assay kits are generally relatively inexpensive, require less expertise, feature low-complexity workflows, and can often be adapted to high-throughput processes.
Biocompare’s assay directory lists thousands of different assay kits, including immunoassays, quantitative analyte assays, enzyme activity assays, and cell-based assays.
Our Assay Kit Search Tool allows researchers to compare assay kits side-by-side and narrow down results by detection target, species reactivity, application, conjugate, or manufacturer. Here, we also provide an assay kit overview guide to help you select the assay kit that best suits your unique research needs.
Types of assay kits
The variety of existing assay types reflects the great diversity of biological analytes and the different ways they can be measured.
A popular method is the immunoassay, which uses specific antibody-binding to detect or quantify target antigens such as proteins, pharmaceuticals, and other metabolites.
This broad application can come in many formats and procedures; common examples include ELISA, chemiluminescent immunoassays, immunoprecipitation, and multiplex immunoassays.
Other assay categories include enzyme assays, protein assays, and cell-based assays.
ELISA kits
In the enzyme-linked immunosorbent assay (ELISA), a target analyte is immobilized in the wells of a microtiter plate by chemical coating or by capture antibodies.
Detection antibodies, often conjugated to HRP or AP, along with chemiluminescent substrate are used to generate a signal proportional to the amount of analytes in the sample.
Commercial ELISA kits generally contain pre-coated microplates and ready-to-use reagents such as capture and detection antibodies, buffers, standards, and detection substrates.
These kits provide a rapid and reliable means of specific antigen detection for a wide range of biological samples, including lysates, homogenates, supernatants, and biofluids.
Our
guide on ELISA Kits
elaborates on ELISA types, selection criteria, and other helpful considerations.
Multiplex Immunoassay kits
Multiplex immunoassays expand the capability of analyte detection by detecting multiple antigens in a single experiment.
This application, which efficiently maximizes the amount of data generated from a limited sample size, can detect from tens to thousands of antigens depending on the platform.
Multiplex assay kits
often come in the form of color-coded beads in 96- and 384-well microplates, such as those utilizing the Luminex® platform.
Other multiplex formats may come in the form of antibody microarrays on glass slides.
Multiplex ELISA kit formats often feature a panel of targets from specified research areas, such as cytokines, hormones, and neurological proteins.
Immunoprecipitation kits
Immunoprecipitation (IP), utilizes specific antibodies to purify or “precipitate” target analytes out of a complex solution, such as a cell lysate or tissue homogenate.
Generally a preparative application, the antibody-bound targets are detected in downstream methods such as immunoblots or enzyme activity assays.
Immunoprecipitation kits
may contain antibodies covalently linked to solid support (eg. agarose or magnetic particles) or immobilized antibody-binding proteins, such as protein A or protein G.
Enzyme assays
Enzyme assays measure the catalytic activity of a specific enzyme or enzyme family.
These biochemical procedures generally quantify the production of enzymatic byproducts, either directly or indirectly via reactive probes.
While often classified based on detection methods (eg. fluorometric, colorimetric, luminescent) there are a diverse variety of enzyme assay mechanisms.
Examples include substrate peptide cleavage, hydrogen peroxide production, NADH/NADPH production, and reactions with chromogenic substrates.
Enzyme assay kits can serve many research areas, including studying protein characterization, enzyme kinetics, biochemical pathways, disease states, toxicology, and pharmaceutical screening.
Included in a typical kit are enzyme substrates, detection reagents, purified enzymes, reaction buffers, stop solution, standards, controls, and cofactors.
Explore a diverse catalog from our
guide to enzyme assay kits.
Quantitative assays
A variety of
quantitative assay kits
are available to measure biologically relevant analytes including proteins, DNA, RNA, and other small molecule compounds.
Among the molecular targets are amino acids, trace elements, sugars, cofactors, lipids, and a wide variety of metabolites.
Protein assay kits
Protein assays are designed to quantify total protein or specific proteins, generally using colorimetric or fluorometric reactions.
Standard protein assays such as the Bradford assay and the BCA assay are commonly used to determine the concentration of total proteins in a sample.
However, other assay kits can also detect specific proteins or those with modifications.
Protein assay kits generally contain components such as detection reagents, buffers, and protein standards.
Our
overview of protein quantification assays
gives helpful considerations for these kits.
Cell-based assays
Cell-based assays, which measure cellular properties such as viability, proliferation, death, and function, are often used in testing the cellular effects of bioactive compounds.
This is a broad category of assays, utilizing diverse mechanisms and methodologies.
Many assay kits are dye-based, such as the cell viability assays that incorporate chromogenic water-soluble tetrazolium salts.
In the presence of active enzymes, these assays produce a measurable color change that is a characteristic of viable cells.
Others, such as the BrdU cell proliferation assay, operate by incorporating bromodeoxyuridine in cells actively replicating DNA.
Assay methods used in apoptosis assays include peptide substrates for pro-apoptotic caspases and annexin V for apoptotic cell membranes.
Learn more about the various techniques for studying cellular processes in our
guide to cell-based assays.
Considerations for assay kit selection
Focus on the measurement goals.
For any type of assay, users should thoroughly define the specific measurement goals of the experiment.
The desired readout—quantitative, semi-quantitative or qualitative—is both an important factor and as well as a limitation for which assay kits can be used.
For instance, a routine test to check whether a protein is expressed may be fine with a semi-quantitative or qualitative assay.
However, in applications like biomarker discovery, a high-sensitivity quantitative assay of multiple analytes, such as with a multiplex immunoassay, may be better suited.
Choose in alignment with the sample type.
In most cases, the starting sample type can readily dictate which assays will be feasible. Tissue samples, for example, can produce lysates (immunoblots and immunoprecipitation), be sectioned (IHC and ICC), form cell suspensions (flow cytometry), or produce culture supernatants (ELISA).
Other sample types, such as bodily fluids, may have analytes in low abundances that lie below the assay’s limit of detection.
Some assays, such as protein assays, may also be incompatible with detergents that are frequently used in cell lysis buffers.
Refer to the assay kit datasheet to determine which sample types can be reliably accommodated by the assay.
Review the protocols.
Aside from the product datasheet, the protocol is a great information source for prospective assay kit buyers.
The assay protocol explains the assay procedure, type of readout, detection method, sample format requirements, interpretation of results, kit components, throughput, and potential issues for troubleshooting.
By preemptively reviewing the protocol, users can anticipate any unique incompatibilities, such as sample type, reagents, or instrumentation.
This information can be used to assess whether the assay's complexity and requirements align with the research team's expertise and budget.
References
Mason, E. Guide to Immunoassays. Biocompare. 2023 Nov 14 [cited 2024 Sep]. Available from: www.biocompare.com/Editorial-Articles/607943-Guide-to-Immunoassays/
Mason, E. Types of Multiplex Immunoassays. Biocompare. 2023 Mar 21 [cited 2024 Sep]. Available from: www.biocompare.com/Editorial-Articles/594795-Types-of-Multiplex-Immunoassays/
DePalma, A. Quantifying Protein Expression. Biocompare. 2021 Aug 12 [cited 2024 Sep]. Available from: www.biocompare.com/Editorial-Articles/578072-Quantifying-Protein-Expression/
Glicksman M, Li Z, Markossian S, et al., editors. In Vitro Cell Based Assays. In: Assay Guidance Manual [Internet]. Bethesda (MD): Eli Lilly & Company and the National Center for Advancing Translational Sciences; 2004-. Available from: www.ncbi.nlm.nih.gov/books/NBK206715/
Mason, E. Choosing an Immunoassay. Biocompare. 2024 Jun 06 [cited 2024 Sep]. Available from: www.biocompare.com/Editorial-Articles/612853-Choosing-an-Immunoassay/
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