Some people use lab based testing (the most sensitive and accurate approach) while others use infield testing kits. Infield kits get you an answer quicker, but it may not be the one you need!
Also, some service providers only sample a limited number of areas. Often limited to 3 or 4. We aim to sample up to 8 areas per kit. This means on a standard 3 bedroom home, one kit will sample almost every room.
A simple rule of thumb, is to sample as many areas as possible and to use lab based testing. This reduces the likelihood of significant levels of meth going undetected. Conversely, if limited numbers of areas are sampled and non-lab based testing is employed, the risk of missing significant levels of meth increases.