To understand why the Pap smear is not a good diagnostic test requires understanding its physical limitations. It is, after all, merely a smattering of loosely smeared cells on a dry slide, which are subsequently covered with a liquid fixative so that nothing will change until examined by the pathology lab it's sent off to. Many different factors can spoil its tidiness, making the pathologist's job of reading it more difficult. If there is any type of infection yeast , bacterial , etc. there may be inflammatory debris and blood mixed in to obscure the true nature of the cells being examined. Inflammation can also make normal cells look abnormal, causing the Pap to be over read as dysplasia.
Menstrual blood can literally deluge the cells on the slide, making it impossible to read.
If it's under-read, then a lesion goes undetected until the next routine Pap smear. If it's over-read, patients will be subjected to the more expensive colposcopy unnecessarily. We gynecologists love the Pap smear, but it's a love-hate relationship, because we know there's got to be a better way. True, we always have colposcopy up our sleeve, but we would rather have a screen that didn't push us that way unless there were true dysplasia to diagnose. That better way has arrived as the ThinPrep system.