Basically, almost all (~99.9%) of the results of tests like 23andme (SNP arrays) are correct, but because an interesting/scary variant/mutation is about as rare in the population as the test result being wrong for a given site, somewhere around half of those scary results are false positive. If 1000 people are tested for a mutation with this method, 10 will be false positive. The real frequency of mutations vary by site - some will be close to 50%, others at 10e-9, depending on how strongly that variant is selected against, so the most harmful ones are very rare. If you get a result from 23andme that your doctor thinks is worth retesting, it's probably because it's a harmful mutation, which is rare, and ergo there's a fair chance the 23andme result is wrong. So 23andme is almost always right, except when the result might affect your health in a very serious way. I should point out that for late-onset diseases like cancer, heart disease, alzheimer's, etc., there's little natural selection, so pathogenic variants are more common, making it more likely that the 23andme result is correct.
It's a simple bayes rule result. It would not be hard for 23andme to give you an estimate of how likely the result is to be a false positive, but maybe there's some regulatory barrier in the way?
jstandard|7 years ago
It sounds like his doctor is quoting this study, which does indeed suggest a 40% false positive rate: https://www.nature.com/articles/gim201838
mmmrtl|7 years ago
Basically, almost all (~99.9%) of the results of tests like 23andme (SNP arrays) are correct, but because an interesting/scary variant/mutation is about as rare in the population as the test result being wrong for a given site, somewhere around half of those scary results are false positive. If 1000 people are tested for a mutation with this method, 10 will be false positive. The real frequency of mutations vary by site - some will be close to 50%, others at 10e-9, depending on how strongly that variant is selected against, so the most harmful ones are very rare. If you get a result from 23andme that your doctor thinks is worth retesting, it's probably because it's a harmful mutation, which is rare, and ergo there's a fair chance the 23andme result is wrong. So 23andme is almost always right, except when the result might affect your health in a very serious way. I should point out that for late-onset diseases like cancer, heart disease, alzheimer's, etc., there's little natural selection, so pathogenic variants are more common, making it more likely that the 23andme result is correct.
It's a simple bayes rule result. It would not be hard for 23andme to give you an estimate of how likely the result is to be a false positive, but maybe there's some regulatory barrier in the way?