Multiple factor indices of protection or risk towards disease

In order to combine several factors entailing protection or risk towards disease and to calculate a Protection Multiple Factor Index (PMFI) or, conversely, a Risk Multiple Factor Index (RMFI), we propose the following formulae: (1) PMFI = 2/[(mF)(2)+1] and (2) RMFI = 2/[(imF)2+1], where mF is the mean value of the factors considered and imF is the inverse (or reciprocal) of mF. In calculating mF, the value of each 'risk factor' observed in the patient under study (Vp) is expressed by taking the mean normal value (Vmn) as the unit, i.e. by calculating the ratio Vp/Vmn, whereas each 'protection factor' is expressed as the reciprocal of this ratio, i.e. as Vmn/Vp. The 'weight' of the various factors can be changed through multiplication by a number >1 or <1. Values of both PMFI and RMFI are always close to 1 in normal subjects, with extreme variations among patients between 0 and 2. The sum of the values of PMFI and RMF1 is always equal to 2, so that one index can be deduced from the other. When factors are only two (F1 and F2), the formulae may be simplified as follows: PMFI = 2/[F1 x F2)+1] and RMFI = 2/[(iF1 x iF2)+1], where if = 1/F, with only minimal changes in results. (C) 2001 Harcourt Publishers Ltd.