When used by an experienced technician, they are about 98% accurate. When used by consumers, the accuracy is about 75% because people often misunderstand or fail to follow the instructions included...
Ironically, the Walmart cheapies -- Equate brand. It identified my pregnancy four days before my period was due. (A faint line, but definitely a line.)
Pregnancy tests that can be bought at the store are generally fairly accurate, if done correctly and within the proper time spans. There is, however, still a chance of a mistake, like a false...