What about 23andMe's SNP test gives it such bad efficacy as a diagnostic tool?
The long and short of it is that genetic variation is actually not very predictive in comparison to "environmental" effects such as lifestyle.

Only a quarter of the variation in lifespan between twins is attributable to inherited factors (including genetics and epigenetics) [1] - the rest is environmental, from lifestyle to air quality.

Most genetic variants (such as those measured on the chip used by 23andMe) have a very small impact on outcomes such as "Risk of disease X, Y or Z". Many risk loci have been identified, but without a full understand of what this risk means, the results of the 23andMe analysis can be misleading.

Some few rare genetic variants have a larger impact on diseases (such as early onset diabetes), but this comes under the "monogenic" category of genetic variation, rather than the common variants typically observed in large populations.

Basically, genetic risk scores are, at the moment, only indicative of an individuals risk, and to get the real picture a lot more information is required - for instance smoking is a better predictor of early mortality than any "common" (>=5% frequency in a population) genetic variants, likewise a high fat diet is more indicative of cardiovascular disease.


Skytthe A, Pedersen NL, Kaprio J, et al. Longevity studies in
GenomEUtwin. Twin Res. 2003;6(5):448–454. [PubMed link]