PubMed · 9529509
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G D Murray. 1998. Statistical commentary.. https://pubmed.ncbi.nlm.nih.gov/9529509/
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Investigators often report results of studies comparing the proportions of subjects who have clinically meaningful responses to various therapeutic regimens. When the outcome variable is a continuous measure this involves dichotomizing the observed response based on a predefined threshold value. The effect of this strategy is examined, paying particular attention to the impact of measurement error on the resulting estimates of treatment effect (difference in the proportion responding). Expressions are obtained for quantifying the magnitude and direction of the resulting bias, and these are illustrated in a study evaluating a pharmaceutical treatment for osteoporosis.
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Concurrent African trypanosome and gastrointestinal helminth infections are prevalent in sub-humid savannah where they are endemic. However, acquired resistance in animals varies with their responder status and exposure. As a guide to study in the definitive hosts, the effects of Trypanosoma congolense infection on the development and maintenance of homologous Heligmosomoides polygyrus resistance were investigated in outbred TO mice. These mice were immunised by abbreviation of larval infection. Immune or naive mice were either infected with 500 infective larvae (L3) of H. polygyrus and/or 10(4) bloodstream forms of T. congolense or were not infected. The outcome of infection was monitored by routine parasitological and immunological techniques for 30 days after the day of the T. congolense infection. Significantly more immune mice concurrently infected with both parasites survived than did immune mice in which H. polygyrus was superimposed on a 10-day-old T. congolense infection. Although all the mice in this latter group died before the end of the experiment, larval immunisation prolonged their survival, relative to similarly treated naive mice. The antibody titres to H. polygyrus in the sera of immune mice challenged with H. polygyrus alone were significantly higher than those of immune mice concurrently infected with both parasites but the levels of protection obtained were comparable. It is concluded that T. congolense may not completely block the strong acquired resistance induced by abbreviated H. polygyrus larval infection in TO mice but is capable of interfering with protective responses, especially if the trypanosome infection occurs prior to H. polygyrus challenge infection.