I have personally done many hundreds of capillary tube clotting time estimations, using homemade glass capillary tubes of a certain size. Gradually, by trial and error, and by chance, ways have been found to reduce or eliminate some of the variables in the test. Unexpectedly, one of the most important variables was found to be in the capillary tubes themselves. This led to a study of the behavior of homemade capillary tubes when used to test the clotting time of normal blood at body temperature, and to the observation, many times repeated, that freshly drawn capillary tubes give short and erratic clotting times, but that tubes from the same batch, after being allowed to age for some months, give very uniform and dependable clotting times, and continue to do so indefinitely. It was also possible to demonstrate that there is only one normal clotting time for a given set of
McNICKLE HF. The Prevention of Primary Postoperative Thrombosis: A Reliable Capillary Tube Clotting Time Method. AMA Arch Surg. 1959;79(5):775–784. doi:https://doi.org/10.1001/archsurg.1959.04320110077013
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