People have been consuming bread for hundreds of years, and it’s still as prevalent as ever. Likewise, ELISA testing, which got its big push into clinical laboratory practice in the 1970s and ’80s, is still considered the tried and trusted method for detecting clinical bio-markers. Although several additional and useful technologies have been introduced over the years, there are many good reasons why ELISA hasn’t disappeared.

ELISA (enzyme-linked immunosorbent assay) uses the concept of specific antigen-antibody complexing to attach an enzyme label to the analyte of interest (usually, but not always, a specific protein).  The substrate is then added to generate color measured by a photometer, or chemiluminescence measured by a luminometer. Using controls or standards these tests can be either qualitative or quantitative.  Chemiluminescent labels extend sensitivity making them especially useful for the detection of analytes in low concentrations such as tumor markers, certain drugs, and hormones.

One reason ELISA testing has maintained its place in the laboratory for so many years is the numerous varieties of ELISA methods available.  Sandwich assays (speaking of bread), direct and indirect methods and competitive assays each offer unique advantages affecting cost and throughput.  In addition, ELISA kits are offered in a wide variety of platforms. Originally performed with coated tubes, beads, and discs, the most widely used format has been the coated microwell.  Several companies produce microwell plates with unique optical and binding qualities, flat or round bottoms, solid or separatable, and having from 96 to 1536 wells per plate. Today large libraries of very specific antibodies and biomarkers are commercially available for ELISA kit manufacturing. Numerous blocking agents and other techniques allow products to be designed with very high levels of specificity and sensitivity. Moreover, ELISA applications can be designed to detect a single very specific protein or a group of proteins having collective biomedical significance.  Arrays and spot ELISA assays results are photographed for software interpretation. All of this versatility extends ELISA application well beyond clinical uses into pharmaceutical, food and beverage quality and safety, and other life sciences.

 

Cost, safe handling, and practicality have also been major factors. ELISA was initially adopted to avoid the hazards of radioisotopes replacing RIA (radio-immunoassay). Most modern ELISA kits use recombinants instead of native antigens.  This not only improves uniformity and reduces biohazard exposure; but can also enhance specificity and robustness for longer stable shelf life and flexible shipping conditions.  Automation for ELISA is relatively low cost with an abundance of commercially available options. Many labs have switched their high volume immunoassays from running in batches in microwells to random access applications on their chemistry and immunoassays systems such as Abbott’s Architect®, Siemens’s Atellica® and ADVIA®, and Roche’s Cobas®. Because it isn’t always practical to run the lower throughput assays on these systems, there is a current trend for high throughput labs to move from 8-12 plate closed systems to 4- plate, 2-plate, and even single plate open system processors with more comprehensive software and flexible options for the assays they run off-line. Another recent trend is to combine several technologies on a single instrument such as microplate processors manufactured to read both photometric and chemiluminescent assays or instruments that can process both spot assays and typical microwell ELISA.

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ELISA remains a classic tool for diagnostics, industry, and research.

The newest 2-plate ELISA processor released by Awareness Technology (USA) handles both colorimetric and chemiluminescent ELISAs even when assays with different protocols are set up in the same microplate.

Each new technology has its place, advantages, and disadvantages; however, ELISA generally requires less training and equipment than methods such as MASS Spectrometry. ELISA results can be faster, more precise, and easier to perform than rt-PCR. Due to its history and high precision, ELISA is often the reference method for cross-validation of new techniques.  ELISA continues finding a place in rapidly growing markets such as biosimilars and cutting-edge proteomics as new biomarkers are discovered and converted into clinical applications.  Labs will certainly want to optimize for more modern applications but ELISA capability isn’t likely to disappear in the foreseeable future.

Expanding Utility of Microplates and Microplate Instrumentation

 

Cost-effective microplate instrumentation is readily available worldwide with a range of products from the single strip reader to high throughput automation with plate transportation, stacking, and storing.  One of its greatest advantages is the widespread use of a standard footprint for the microwell plate.  First developed in 1951 to replace test tubes in mass screenings, the traditional plate consisted of 12 rows of 8 wells each.  Manufacturers have developed a large variety of alternate microwell sizes and depths to accommodate other technologies while conserving the base dimensions.  To further extend the utility of existing microplate-based equipment other technologies including gels and slides have been designed for use with carriers that preserve the same footprint and sample spacing. Therefore, labs can strategically modify older or modular equipment for use with new technologies, bringing them to fruition faster and with a lower capital investment.

Perhaps the best example is the use of deep well plates. These are used in a range of applications including fraction collection, sample dilutions, mixing, and gene amplification processes.  Most liquid handling systems originally designed for lower profile wells have been adapted not only for mechanical handling but to support the other conditions of these technologies such as temperature control and sterility. Some deep well plates include cutouts on the base for easier transportation by robotic systems.

According to a recent market survey, the global microplates market is set to reach US$ 1268.5 Mn by 2029 from US$ 729.7 Mn in 2019 at a compound annual growth rate (CAGR) of 4.2% during the forecast period from 2021 to 2029. The growth is less significant in the clinical sector and mostly due to the increase in microplate-based biotechnology research largely because microplate processing equipment has proven to be so efficient and precise for handling, labeling, processing, and storing large numbers of samples.  The 384-well microplate configuration, originally introduced in 1994, currently leads the market with its ability to perform in numerous applications including miniaturized gene transfer assays.  These low-volume wells (120uL total volume) reduce the cost of experiments while simultaneously providing high throughput.  Manual and automated multi-tip pipetting devices are commercially available to dispense from 1 to 100uL per well.

A 40-year-old American designer of laboratory instrumentation, Awareness Technology supports the industry with both the clinical and research lab requirements for incremental adaptations of microplate technology.  Its first instruments in the 1980s were designed for photometric ELISA applications.  Today the expanded product line includes many OEM developments based on the tried and trusted microplate.  An economical liquid handling system was recently prototyped to support large-scale PCR testing for COVID 19 screening.

Another recent example is the completely automated system for RPR screening (syphilis).  This device replaces a once tedious manual process having subjective visual interpretation with a 4-plate completely automated processor that photographs test results for objective software interpretation.  

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Agglutination assays automated using four 48-well plates

Microwell ELISA is only one of many uses for microplate processers, readers, and washers today. These devices, with the continual updating of robotics, electronics, and software, are rapidly aiding advances in proteomics, genetics, and other scientific frontiers.