The standardization of tuberculin / C.C. Okell and H.J. Parish.

  • Okell, C. C. (Charles Cyril), 1888-1939.
Date:
[1927?]
Catalogue details

Licence: In copyright

Credit: The standardization of tuberculin / C.C. Okell and H.J. Parish. Source: Wellcome Collection.

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    An initial attempt was made to place samples of tuberculin of different values and the standard tuberculin in their correct order. This was done with 5 series of 4 tuberculins, 50 guinea-pigs being used for the experiments. If the tuberculins differed 40% or more in potency, it was nearly always possible to place them in their correct order by the test. Taking 40% as the difference between two tuberculins that was just clearly detectable, a further series of tuberculin preparations was compared and percentage values allotted to them (Table I). The values estimated from the test are not to be taken as more than attempts to assign a percentage, but it will be seen that the attempts were almost always good ones. Only once in 20 comparisons was the estimated value further out than 30%, and it was usually a good deal less. Considerable numbers of guinea-pigs had, however, to be used in order to arrive at this degree of accuracy, and it must be clearly understood that the reading of individual guinea-pigs may give values that are widely aberrant. For practical purposes 4 to 6 guinea-pigs will indicate differences of 40 per cent, with a fair degree of certainty, and with the use of a greater number of animals considerably finer differences may be detected. It may be added that the average standard deviation of the series of values given in Table I corresponds to an error of 22*2 per cent, of the activity of the sample tested—that is, the expected error would be rarely above 44*4%, which agrees with the rough estimate first arrived at. QUANTITATIVE VON PIRQUET TEST IN GUINEA-PIGS. Table II gives the results of 16 comparisons of tuberculin preparations with the standard tuberculin. It will be seen that this method gave much less satisfactory results than the intradermic method. Occasionally good estimates were made, but in many cases the estimate was very inaccurate. The chances of being able to detect 50% differences are small, and it is possible to make such a gross error as to state that a given sample is 50% of standard strength when in reality it is tuberculin-free (e.g. sample Kl). With weak tuberculins, also, very wide errors can be made, as the maximum reactions produced on the guinea-pigs may not be good enough to be read with ease. On individual guinea-pigs the reactions due to standard tuberculin, undiluted, and diluted 1 in 4 and 1 in 8, may all be indistinguishable. To get the maximum accuracy with this test the tuberculins and their dilutions must give well-marked and well-graded reactions. To sum up, if the tuberculins are sufficiently strong, the guinea-pigs highly sensitive, and the gradation of the reactions good, 50% differences are often detected by the von Pirquet test, but such wide errors may be made that the test may be considered as useless for the purposes of tuberculin standardization. SUBCUTANEOUS TEST IN GUINEA-PIGS. The figures for this test were arrived at by a somewhat different method. As each batch of guinea-pigs became appropriately sensitive, it was subdivided into four groups, which were injected, not with unknown dilutions, but with