WOAH Concludes Evaluation of Bovine Tuberculin Candidates  

WOAH Biological Standards Commission decides to relaunch a global call for new tuberculin candidates after extensive international trials

Following a decade of international studies, the WOAH Biological Standards Commission, supported by an ad hoc group of experts, has concluded that the proposed replacement candidates for the International Standard for Bovine Tuberculin do not meet the required potency levels. It will now relaunch a global call for new candidates’ materials. 

What is the International Standard for Bovine Tuberculin? 

The International Standard Bovine Tuberculin (ISBT) is a biological reference material derived from Mycobacterium bovis strain AN5. Established nearly three decades ago, it defines a standardised protein concentration that serves as the global benchmark for tuberculin potency testing.  

ISBT is used worldwide in bovine tuberculosis control programmes for screening cattle. All commercial bovine tuberculin preparations must be calibrated against this standard. 

The science behind tuberculin testing 

Tuberculin testing exploits the immune system’s memory of infection. When Mycobacterium tuberculosis complex organisms infect cattle, the animal’s immune system develops a hypersensitivity response to specific bacterial proteins. The tuberculin skin test capitalises on this biological activity by injecting purified protein derivative (PPD) intradermally and measuring the resulting immune response as an increase in skin thickness. This delayed hypersensitivity reaction, measured 72 hours post-injection, indicates prior exposure to tuberculosis. 

A decade-long effort to replace the standard 

The current ISBT, established in 1986, has been in limited supply since 2015. In response, WOAH’s Biological Standards Commission, supported by an ad hoc group of mammalian tuberculosis reference laboratory experts, has coordinated experiments with multiple laboratories, research institutes and tuberculin manufacturers across continents to identify a suitable replacement. 

Key findings  

Two candidates were initially shortlisted in 2017. Following further evaluation, only one (Candidate B) progressed to international collaborative trials conducted between 2018 and 2025.  

However, the latest assays revealed persistent potency shortfalls: 

• Potency levels (18,000–19,116 IU/mg) remained well below the required minimum of 32,000 IU/mg. 

• Multiple laboratories confirmed inconsistent results despite protocol refinements. 

• High assay variability and weak dose-response relationships hindered reliable potency determination. 

Final trials in November 2025 confirmed that Candidate B does not meet the criteria to replace ISBT. 

Future directions 

The Biological Standards Commission acknowledged that PPD is a complex biological substance having many specific and non-specific proteins. They emphasised that accepting a lower potency international standard would compromise the comparability of global tuberculin standardisation across laboratories and batches.  

The Commission therefore decided to: 

• relaunch a global call for new tuberculin candidates. 

• support the development of alternative approaches for PPD-based tests such as the Molecularly Defined Tuberculins (MDTs) which use specific peptides as a long-term alternative to traditional PPD-based assays. 

While promising, MDTs require further field validation across different epidemiological settings before they can replace current methods.

More information 

The full meeting report, including detailed methodology and statistical analyses, is available on the WOAH website: 

WOAH Ad hoc Group on Replacement of the International Standard for Bovine Tuberculin. Report 2015-2025.

Ad hoc Groups – WOAH – World Organisation for Animal Health 

Biological Standards Commission – WOAH – World Organisation for Animal Health 

Contact: [email protected]