parasite testing is very hit or miss and seldom do people receive positives. Clinicians are trained in specific identification and if the sample does not meet the qualifications then you get a negative. There are many species of parasites yet to be determined to be pathogenic so these do not show up in handbooks. Worms are generally identified by way of the ova which have much variance among worms. This means they do not pay attention to the adult but rather search for ova then match that against their manuals.