What Are Diagnostic Reagents?

Lucy Bell-Young

by Lucy Bell-Young

2nd June 2021

Diagnostic reagents are chemicals used in laboratories to determine specific types of pathogens, metabolic abnormalities, physiological anomalies, and genetic diseases. They’re relied on by medical practitioners to make accurate diagnoses, and can be used in vivo or in vitro in order to detect certain diseases. 

Diagnosis is key in curing and treating medical conditions. Without dependable diagnostic tools, the medical field would be ineffective and unreliable. For example, if physicians were unable to correctly determine the medical problem, they’d be unable to recommend the proper treatment. In this way, diagnostic reagents are crucial in ensuring the accurate diagnosis of diseases.

What is the Definition of Diagnostic Reagents?

Diagnostic reagents are any organic or inorganic compounds that are added to an analyte or biological specimen, like urine, a blood sample, or biopsied human tissue, to determine pathogenesis or abnormalities.

Doctor holding diagnostic reagents

They’re typically organic compounds that either react directly with the sample being tested, or act as catalyst for a chemical reaction that indicates the presence of certain pathogens or abnormal metabolites. In this way, they work the same as ‘normal’ reagents, which are added to a solution to either cause a chemical reaction, or to test if a reaction has occurred.

For example, in testing for SARS-COV2 infection, the most common type of test used is the RT-PCR, a.k.a the real-time reverse transcription polymerase chain reaction test. The reagents used here are enzymes that detect the genetic materials specific and unique to the virus.

Diagnostic reagents shouldn’t be confused with assays. An assay is a broader term that refers to an investigative or analytical chemistry method used in medicine, pharmacology, mining, molecular biology, and ecological sciences to test for the presence of certain chemicals or entities in a sample material.

Diagnostic reagents, on the other hand, specifically test for diseases and other medical conditions, making them a sub-component of assays.

Want to know more about reagents? Read our article on Different Types Of Chemical Reagents And Their Uses

What Are Medical Diagnostic Reagents?

Medical diagnostic reagents are reagents used for medical diagnosis. They’re usually ordered by physicians to make sure of their diagnosis. For example, if a patient is showing symptoms of dengue fever, a physician can order a blood exam, which involves counting the platelets and testing for certain markers.

Medical diagnostic reagents can be classified into three major categories based on the EDMA (European Diagnostic Manufacturers Association) classifications:

  1. Clinical chemistry or medical biochemistry: The reagents classified under this category are used for testing various body functions, such as in the kidneys and liver. The reagents test for biochemical markers that indicate either normal or abnormal functions. Some reagents are also used for detecting toxic substances and pathogens.
  1. Immunochemistry: Immunochemistry reagents are used for testing the immune response of the body, such as allergic reactions. They can also detect tumour markers and cardiovascular disease markers.
  1. Hematology: These reagents are used to test the blood and blood serums. The most common types of reagent used in hematology are those for testing blood types.
Cropped closeup of blood test tubes arranged on a tray
Reagents are used in hematology to test blood samples

What Are Clinical Diagnostic Reagents?

Clinical diagnostic reagents are a subcategory of medical diagnostic reagents. They include enzymes, electrolytes, substrates, and multiple-parameter reagents, and are used to detect specific biochemical markers that indicate certain types of pathological conditions.

For example, using enzymes to measure the cholesterol level in a sample blood serum is a useful indicator for the following:

  • Liver function
  • Biliary function
  • Intestinal absorption
  • Propensity toward coronary artery disease
  • Thyroid function
  • Adrenal disease

The methodology for the quantitative determination of cholesterol in a human blood serum is:

Methodology for the quantitative determination of cholesterol in human blood
  • Step 1: Cholesterol esterase is used to hydrolyse cholesterol esters, turning the latter into cholesterol and fatty acids
  • Step 2: Cholesterol oxidase is used to oxidise the free cholesterol, turning it into cholest-4-en-3-one and hydrogen peroxide
  • Step 3: Chromophore (quinoneimine dye) is formed by combining hydrogen peroxide, HBA and 4-aminoantipyrine. This dye can then be used in the analysis

Diagnostic Reagents Examples

Various diagnostic reagents have different brand names, but may refer to the same reagent. They can be bought either as part of a kit or as individual products. They can either be for laboratory use only, or for individual use in non-laboratory settings, like a home pregnancy test kit.

Pregnancy test kits

A home pregnancy test kit uses immunochemistry reagents to detect human chorionic gonadotropin (HCG). HCG is the hormone produced by the placenta of a woman when an embryo has implanted.

Between 97-99% accurate, this reagent test kit is a relatively simple tool that doesn’t require any advanced training in medicine or biochemistry to interpret the results. This makes it incredibly accessible and the easiest way to test for pregnancy. 

The method involves getting a urine sample on the stick, and then looking at the lines that are formed on the test window afterwards. Typically, if two lines are formed, it means that HCG is high enough to be detected in the urine, which is indicative of pregnancy.

Alcohol breathalysers

Breathalysers are alcohol detection devices. There are three major types used by traffic police, all of which differ on some key components and working principles. Two types use a reagent as a reactant to alcohol:

  • Simple breathalyser: This detects alcohol through a reagent that reacts with alcohol and produces a colour change
  • Intoxilyser: This type of breathalyser relies on electronics, particularly infrared (IR) spectroscopy
  • Alcosensor III or IV: If alcohol reacts in a reagent in a fuel cell, the reagent detects it, indicating the presence of alcohol

The portable alcohol breathalyser is another example of a testing kit that doesn’t require complex laboratory processes or expert-level skills to interpret the results. It’s simply the combination of electronics and chemical reactions that allows for alcohol detection in the breath.

Alcohol is easily absorbed into the intestines and becomes part of the blood. Trace amounts of alcohol vapour, however, is left in the lungs and stomach. This is what’s detected by the breathalyser. If it reaches a certain level, a proportional level can be calculated to be present in the blood.

Alveolar air that contains a certain level of alcohol can be measured based on a standard limit. For many jurisdictions, the allowed limit is below 0.08 grams per 100 mL of blood. The alveolar air alcohol content proportion to blood is 2,100:1. This means that for every 2,100 mL of alveolar air, the proportion of alcohol in the blood will be the same per 1 mL of blood.

Closeup of man placing the sample into the covid-19 antigen diagnostic test device

COVID-19 rapid tests

Rapid tests, or antigen tests, are typically used by mobile laboratories and healthcare workers in the field. These tests are commonly available and administered in many public places like testing centres, clinics, and airports. The result can be known within a few minutes, but the tests aren’t as accurate as the standard RT-PCR tests.

As the name suggests, the antigen tests detect the presence of viral antigens produced by the body. The test kits have reagents in them that react with a particular class of antigens that are naturally produced by the body against coronaviruses. This means false positives may be detected if a person is infected by a common cold virus.

Diagnostic reagents are very useful in aiding physicians to conclusively identify a disease, ailment, or medical condition, allowing for the right treatment to be administered. For example, the reagents used in COVID-19 tests made it possible for simplified portable test kits to be made. Currently, these reagents are among the many weapons we have in our battle against the pandemic.


All content published on the ReAgent.ie blog is for information only. The blog, its authors, and affiliates cannot be held responsible for any accident, injury or damage caused in part or directly from using the information provided. Additionally, we do not recommend using any chemical without reading the Material Safety Data Sheet (MSDS), which can be obtained from the manufacturer. You should also follow any safety advice and precautions listed on the product label. If you have health and safety related questions, visit HSE.gov.uk.