Minimal mapping quality as parameter to call a variant

[felipe.vaca]

Hi,

I am working with Solid 5500XL in exome sequencing profiling from blood-tumour pairs, using Lifescope to align and do the variant calling. Among other paramenters, in Lifescope the minimal mapping quality value to call a mutation is 8 (using the default parameters). Considering that this value is in Phred scale, the probability of error in the alignment is 0.1584. This seems to be a very high probability of error.

QV=8
P=0.158489319
Acuracy(%)=84.15106808

I wonder why is this value so low as compared to base quality values used for the variant calling (Phred=26). Which mapping QV do you use as cutoff?

Thanks in advance,

Felipe