Testing Milk for rbST

Dr. Normand St-Pierre, Dairy Management Specialist, The Ohio State University (top of page)


  • Tests based on monoclonal antibodies cannot distinguish rbST in the presence of the four other variants in milk. The theoretical concentration of rbST in milk under normal use is so low and the rbST molecule is so similar to the non-recombinant variants, that it is unlikely that any antibody-based method, especially monoclonal, will ever be successful at detecting milk from cows supplemented with rbST.
  • A test based on changes in the fatty acid binding protein (FABP) appears unlikely to ever differentiate milk from rbST supplemented cows because so many factors affect the maintenance of mammary cells, and thus, FABP.
  • It appears unlikely that a rapid, accurate, and sensitive test for detecting rbST supplementation in dairy cows using milk samples will be derived anytime soon.


  1. A paper by Erhard et al. (1994) based on a possible antigenic difference of recombinant and pituitary bovine growth hormone raised the possibility of using monoclonal antibody techniques to test for the presence of rbST in serum and milk of dairy cows. Unfortunately, the technique cannot distinguish recombinant bST in the presence of the four other variants in milk. More recent work has lead to the same conclusion. For example, a recent paper by Castigliego et al. (2007) showed the possibility of testing for some forms of rbST using immunodetection with a sandwich ELISA. The authors, however, concluded that the method's "discrimination ability still cannot provide support for any lawsuit, confirming the difficulty in immunologically discriminating rbST prom pituitary bST, especially if recombinant molecules with extremely reduced differences in primary sequence are involved. In fact, the more recent commercialized molecules overlap with one of the major natural NH2-terminus variants, representing a considerable problem in making an immunologically based assay that will discriminate effectively".

    The challenge of developing a test to specifically distinguish rbST in milk is several times more complex than finding any bST due to the fact that there are four natural forms (variants) of bST produced by dairy cows. The rbST in Monsanto's Posilac is derived from one of these variants.

    Monsanto did not put a "tag" on rbST. There is a methionine amino acid in the # 1 position on Monsanto's rbST molecule. It appeared there as a result of the recombinant process and was not removed because it had no impact on biological activity. Because all cows have a minimum of either 2 or 4 variants of bST (and there are actually more variants based on research conducted at Ohio University in the 1990's), each of two different lengths (190 or 191 amino acids), the methionine in the #1 position on Monsanto's rbST molecule can be viewed as either an additional amino acid to one of the 190 amino acid molecules resulting in a 191 variant, or a substitution of the first amino acid in one of the 191 variants. It is not involved in binding or the 3-D structure of the molecule. Because the methionine in the #1 position of the rbST molecule is in a non-biologically active portion of the molecule and does not change the bioactivity of the rbST molecule from its parent bST molecule, the FDA did not require its removal.

    The fact that the #1 position is a non-biologically active part of the molecule makes it EXTREMELY difficult to detect any changes that occur there. The challenge is trying to find a miniscule difference in an infinitesimally small amount. The amount of bST in raw milk is so small, (0.5 ng/ml, or 500 parts per trillion; Schams, 1990) it approaches the lower-limit of being detectable at all.

  2. More recently, a patent was issued in 1997 to Dr. R. C. Gorewit of Cornell University for a test based on associated changes in a protein referred to as FABP (fatty acid binding protein). The test is based on comparing rates of phorphorylation of FABP. This involves the isolation of globular membranes that surround the milk fat droplets and purifying them by column chromatography techniques. The FABP fraction is then collected and concentrated by ultra-filtration. Finally, samples of the FABP preparation are incubated with radioactive phosphate (gamma 32P-ATP) and the extent of radioactive 32P incorporation is determined. The basis for this test has since been published by Spitsberg and Gorewit (2002). Dr. Gorewit ideas were somewhat speculative and were based on very limited work. Neither he nor anyone else has demonstrated an actual relationship between FABP in milk and use of rbST.

    The FABP is related to maintenance of mammary cells, and thus, varies widely. Factors such as milk yield, persistency of lactation, stage of lactation, pregnancy, parity, breed, diet, season, environmental temperature, and animal health all affect the maintenance of mammary cells. All of these factors would give expected changes in FABP similar to what is speculated to occur with rbST. This patent has since expired due to the owner of the patent failing to pay the renewal fees.

    Any test for rbST would require validation, including reasonable estimates of repeatability, sensitivity, variability, and accuracy. None of these have been reported for the FABP hypothetical method, and it is doubtful whether the actual methods used for FABP could ever meet reasonable minimum standards.

In spite of the huge economic incentive to develop a test for detecting milk from cows supplemented with rbST, such a test has not emerged and is nowhere close on any radar screen. Thus, it appears highly unlikely that a rapid, accurate and sensitive test for the detection of rbST in milk will be derived anytime soon. Also, because all constituents in milk from rbST supplemented cows are in the normal concentration ranges found with non-supplemented cows, it appears very unlikely that a rapid, accurate, and sensitive test to detect rbST use from constituents in the milk of rbST supplemented cows will be found in the near future.

(References are available on request.)