All medical tests have some margins of error, some quite little and some significant. Inaccurate and unreliable testing remains a serious health hazard, as well as waste of money. Faulty tests can occur for many reasons: a machine loses its calibration or may be faulty; testing chemicals lose potency or get used improperly; human specimens are inadvertently switched. Human errors by those administering the test, those doing the laboratory work compound the potential mistakes. Even if a test is performed properly, it may be misinterpreted, especially if the screeners are overworked. Patients and doctors tend to view such tests as infallible or with little error. Treatments resulting from these diagnosis can be quite disastrous.
Boston researchers studied 10 women with cervical cancer who had negative test results in the preceding two years. A re-examination of their slides found out that five had been misinterpreted and two slides were too poorly done to read; only three were clearly negative. Monitoring of the 4.5 million smear tests taken each year in the UK estimates that 93% were processed accurately. Examples of incorrect processing which may lead to errors include: using the wrong spatula, or a finger instead of a spatula, to take the cervical cells, and incorrect analysis procedures and insufficient re-testing procedures in the laboratory. At one hospital, almost 20,000 smears taken between 1987 and 1992 had to be rechecked because a laboratory technician had misread microscope slides.
In 1995 it was reported that charges of reckless homicide had been brought against a medical laboratory that had been accused of misreading Pap smears of two women who later died of cervical cancer. The case was the first in the USA in which a laboratory had been charged with a crime because of an error.
In 1999 it was reported that medical mistakes killed from 44,000-98,000 people each year in the USA, of which 7,000 were caused by medication errors.