Febrile Antigens are stained bacterial antigen suspensions used to identify and measure antibodies in human sera, following infection. Bacterial infection will cause the body to produce antibodies in response to the invading pathogen. When the serum sample is mixed with a febrile antigen, the solution will agglutinate if antibodies are present, indicating infection. Febrile Antigens are also known as Widal reagents – named after George-Fernand Widal who applied the agglutination reaction (discovered by Gruber and Durham) to test for typhoid fever using human serum. The Widal test became the first example of serum diagnostics, leading the way for blood science.

Widal agglutination tests are widely used in many developing countries, including India, as an alternative laboratory procedure for diagnosis of enteric fever. Baseline titre of antibodies is normally elevated in healthy individuals in endemic areas. This factor must be considered before interpretation of Widal titres. In India, a titre of <1:20 is considered as the baseline of antibody titres against S typhi in normal healthy individuals1.

Rapid Labs offer a range of high density and standard Febrile antigens for the identification of Salmonella, Proteus and Brucella bacterial infections. High density febrile antigens are typically used in areas which have a low exposure to the disease, whilst standard febrile antigens are more suited to endemic areas with a high incidence/risk of the disease. All our tests are easy-to-use, affordably priced and offer fast results to enable efficient screening for disease. They are available to use as both slide or tube methods. The slide method provides immediate agglutination, which is useful for obtaining an estimate titre, whilst the tube method can be used to determine the titre and is generally considered a more accurate (albeit less rapid) method.


Brucella Febrile Antigen Range:

Brucella are a group of Gram-negative, rod-shaped bacteria. Infection (Brucellosis) can occur in humans usually by the consumption of unpasteurised milk and/or close contact with infected farm animals. Rapid Labs offer febrile antigens for two of the most common Brucella species that can affect humans: Brucella abortus and Brucella melitensis. Brucella melitensis is the most virulent and invasive species, usually infecting goats and sheep2. Brucella abortus is less virulent and primarily affects cattle2. Our febrile antigens are available in 5ml volumes:

Product NameTypeCat. No.
Brucella abortus Febrile AntigenHigh DensityHF-BAAF
Brucella abortus Febrile AntigenStandardF-BAAF
Brucella melitensis Febrile AntigenHigh DensityHF-BMA5
Brucella melitensis Febrile AntigenStandardF-BMA5


Proteus Febrile Antigen Range

Proteus is a genus of Gram-negative saprophytic bacteria. They are found in decomposing matter, manure soil, faeces and sewage. The OX-2, OX-19 and OX-k strains can cross-react with several species of the Rickettsiae bacteria3:

Proteus StrainRickettsiae StrainAssociated Disease
OX-19Rickettsia typhiMurine Typhus
OX-19Rickettsia prowazekiiEpidemic Typhus
OX-kOrientia tsustsugamushiScrub Typhus
OX-2Rickettsia rickettsiaRocky Mountain Spotted Fever
OX-2Rickettsia africaeAfrican Tick-Bite Fever
OX-2Rickettsia japonicaJapanese Spotted Fever


Rapid Labs offers Febrile Antigens in 5ml sizes for the OX-2, OX19 and OX-k strains of Proteus:

Product NameTypeCat. No.
Proteus OX-19 Febrile AntigenHigh DensityHF-POX195
Proteus OX-19 Febrile AntigenStandardF-POX195
Proteus OX-k Febrile AntigenHigh DensityHF-POXK5
Proteus OX-k Febrile AntigenStandardF-POXK5
Proteus OX-2 Febrile AntigenHigh DensityHF-POX25
Proteus OX-2 Febrile AntigenStandardF-POX25


Salmonella Febrile Antigens

Salmonella is a genus of rod-shaped Gram-negative bacteria. There are over 2500 serotypes. Rapid Labs offer a range of febrile antigens for the most common serovars tested for in 5ml sizes. Serovars displaying somatic ‘O’ antigens are stained blue, whilst serovars displaying flagella ‘H’ antigens are stained red4. The IgM somatic O antibody appears first and represents the initial serologic response in acute typhoid fever, while the IgG flagella H antibody usually develops more slowly but persists for longer:

Antigen Group SerovarsFebrile AntigenTypeCat. No. 
AS. Paratyphi A, S. Paratyphi A var. DurazzoSalmonella O Antigen Group AHigh DensityHF-SOA5
Salmonella O Antigen Group AStandardF-SOA5
Salmonella H Antigen Group AHigh DensityHF-SHA5
Salmonella H Antigen Group AStandardF-SHA5
BS. Paratyphi B, S. Paratyphi B var. Odense, S. Java, S. LimeteSalmonella O Antigen Group BHigh Density HF-SOB5
Salmonella O Antigen Group BStandardF-SOB5
Salmonella H Antigen Group BHigh DensityHF-SHB5
Salmonella H Antigen Group BStandardF-SHB5
CS. Paratyphi C, S. Choleraesuis, S. Choleraesuis var. Kunzendorf, S. Decatu, S. TyphisuisSalmonella O Antigen Group CHigh DensityHF-SOC5
Salmonella O Antigen Group CStandardF-SOC5
Salmonella H Antigen Group CHigh DensityHF-SHC5
Salmonella H Antigen Group CStandardF-SHC5
DS. Typhi, S. NdoloSalmonella O Antigen Group DHigh DensityHF-SOD5
Salmonella O Antigen Group DStandardF-SOD5
Salmonella H Antigen Group DHigh DensityHF-SHD5
Salmonella H Antigen Group DStandardF-SHD5


If you would like more information about any of our febrile antigens, or would like to place an order, please get in touch and we’ll be happy to help.

Email: info@rapidlabs.co.uk
Tel: +44 (0) 1206 250484



  1. Punia et al (2003). Determination of baseline Widal titres from Chandigarh. Indian Journal of Medical Microbiology 21144 (2)
  2. Alton & Forsyth (1996). Brucella. Medical Microbiology (4) C28
  3. Walker & DH Bouyer (2007). Rickettsia and Orientia. Manual of Clinical Microbiology (9) p. 1042
  4. Serotypes and the Impotrance of Serotyping Salmonella. Centers for Disease Control and Prevention. Last accessed 3rd July 2019 at https://www.cdc.gov/salmonella/reportspubs/salmonella-atlas/serotyping-importance.html