WHO biological reference standards are carefully prepared physical samples that serve as the global measurement benchmark for vaccines, blood products, diagnostics, and other biological medicines. Each standard is assigned a defined potency in International Units (IU), giving laboratories, manufacturers, and regulators worldwide a common language for measuring biological activity. Without them, a test result in Tokyo and a test result in Toronto would have no reliable way to mean the same thing.
What These Standards Actually Are
Unlike chemical drugs, which can be precisely measured by weight, biological products like vaccines, hormones, and antibodies are complex molecules whose potency depends on how they behave in living systems. Two batches of the same vaccine might contain identical amounts of protein by weight yet trigger very different immune responses. WHO biological reference standards solve this problem by providing a physical reference point: a set of ampoules or vials containing a specific biological substance with an internationally agreed-upon activity level.
When a manufacturer or diagnostic lab needs to measure the potency of a product, they test it side by side with the relevant WHO standard. The result is expressed in International Units, which are defined not by an abstract formula but by the actual contents of the reference ampoule. This system ensures that “1,000 IU” of a substance means the same thing regardless of which lab measured it or which assay method they used.
How a New Standard Gets Created
The process starts when a need is identified, whether for a new disease area, a new type of therapy, or a replacement for a depleting existing standard. Any proposal to develop a new WHO biological reference material must first be endorsed by the Expert Committee on Biological Standardization (ECBS), a body established in 1947 that oversees the entire system.
Once approved, a candidate material is prepared and sent to a large network of laboratories around the world for an international collaborative study. Each participating lab tests the candidate using its own methods and equipment. The purpose is not to see if one lab gets the “right” answer, but to characterize how the material performs across many different assay systems and determine whether it behaves consistently enough to serve as a reliable benchmark. The results are compiled into a scientific report and submitted to the ECBS, which makes the final decision on whether to formally establish the material as a WHO standard.
If no previous standard exists for that substance, the new standard is assigned an arbitrary value in International Units. If it is replacing a depleted standard, the candidate is calibrated directly against the existing one, so the IU remains comparable over time. In a strict measurement sense, the IU is always redefined by the contents of the new ampoule, but the calibration process keeps the practical meaning stable across generations of standards.
Categories and Scope
WHO biological reference standards cover a broad range of products. They include standards for vaccines, blood products and clotting factors, hormones, cytokines (immune signaling proteins), antibodies, and in vitro diagnostic reagents. Some standards are straightforward, with a single assigned potency value. Others are more complex. The Third WHO International Standard for low molecular weight heparin, for example, was assigned values for multiple types of anticoagulant activity. The First WHO International Standard for rituximab, a cancer therapy antibody, carries separate IU values for four distinct biological activities, including cell-binding and cell-killing functions.
More recent additions reflect the expanding scope of biological science. A reference panel for specific genetic mutations in the KRAS gene was established in 2017 to support precision oncology diagnostics. In 2022, a reference reagent for gut microbiome analysis was created to check the accuracy of high-throughput DNA sequencing technologies. And in 2023, a reference reagent for antibodies to Ross River virus was established even before any licensed vaccine existed, anticipating the need for standardized measurement as candidate vaccines moved through development.
The COVID-19 Example
The pandemic illustrated why these standards matter in real time. Early in the crisis, labs worldwide were developing PCR tests for SARS-CoV-2, but without a shared reference point, comparing sensitivity across different tests was nearly impossible. The First WHO International Standard for SARS-CoV-2 RNA was developed from an inactivated isolate of the virus (the England/02/2020 strain), treated with acid and heat to render it non-infectious, then validated through serial passage on cells to confirm complete inactivation.
After an international collaborative study, the ECBS assigned it a potency of 7.40 log₁₀ IU per ampoule. Labs could then calibrate their PCR-based tests against this standard, making it possible to compare diagnostic performance across countries and platforms. A similar process produced standards for SARS-CoV-2 antibodies, which became essential for evaluating vaccine effectiveness and comparing immunity data from clinical trials conducted by different manufacturers in different populations.
Who Stores and Distributes Them
The physical standards are held by a small network of custodian laboratories. The largest custodian is the National Institute for Biological Standards and Control (NIBSC) in the United Kingdom, which houses a substantial portion of the WHO catalogue. Other custodians include the U.S. Centers for Disease Control and Prevention (CDC), the National Institute of Allergy and Infectious Diseases (NIAID) at the NIH, the European Directorate for the Quality of Medicines (EDQM) in Strasbourg, Germany’s Paul-Ehrlich-Institut, and the University of Washington’s lipid research laboratories.
Labs that need a standard submit a request directly to the relevant custodian along with a brief description of their intended use. Because these are physical materials, stocks eventually run out. When a standard nears depletion, the replacement cycle begins with a new collaborative study to calibrate the successor material. Discontinuation of a standard is rare, but it can happen if the material no longer reflects current science or has very limited geographic relevance.
How Labs Use Them Day to Day
Most laboratories do not use the WHO International Standard directly for routine testing. International Standards sit at the top of a calibration chain: they are the primary reference. National regulatory agencies, diagnostic manufacturers, and vaccine producers use them to calibrate their own secondary (or “working”) standards, which are then used for everyday quality control and batch release testing.
The calibration process involves testing the candidate secondary standard in parallel with the WHO standard across a minimum of three independent assay runs, using a fresh vial of each standard for every run. The results are analyzed to determine the relative potency of the secondary material, expressed in IU per milliliter. For secondary standards used across multiple labs or countries, this calibration is typically done through a collaborative study. For in-house working standards at a single manufacturer, calibration by that laboratory alone may be sufficient.
This tiered approach preserves the limited supply of WHO standards while ensuring that measurements in diagnostic labs, manufacturing facilities, and clinical trials around the world all trace back to the same reference point. It is, in effect, the biological equivalent of calibrating your kitchen scale against a national reference weight, which itself was calibrated against an international prototype.