PAP SMEAR REVISITED

Unrealistic expectations spawn much flawed medical malpractice litigation. Nowhere is this more true than with popular screening tests.

"The Papanicolaou smear is arguably the most cost-effective cancer screening test ever devised" ¹. As with mammography however, public health promotion of the test has resulted in grossly distorted perceptions of its power to detect and prevent disease. Laboratory error does not equate with negligence.

Practice Point

**Laboratory error is inevitable and is not de facto evidence of substandard technique**

All scientific measurement is subject to error, and the sources of error are particularly complex in biological systems. In medical testing, the range of error must be known and allowed for in interpreting a test result in the context of the clinical situation. Hemoglobin (Hb) measurement is subject to cumulative errors of about 10%. Thus, in approximate terms, a change in measured Hb from 10 to 9 could in reality represent anything from a fall from 11 to 8 on the one hand, to a rise from 9 to 10 on the other.

Practice Point

The very best medical screening tests are subject to significant proportions of false negative and false positive results

In all examinations and measurements to detect the presence or absence of a disease, there is an inverse relationship between the proportion of false positive results (specificity) and the proportion of false negatives (sensitivity). Even in a test with intrinsically powerful discrimination, setting the sensitivity too high results in an unacceptable increase in the number of patients subjected to potentially dangerous further investigation, unnecessary treatment and unwarranted anxiety.

The errors of tests for screening a healthy population tend to be especially problematic, particularly if the probability of the presence of the disease in a given individual is low. The dilemma is that a high proportion of individuals who do not suffer from the disease amplifies the conflict between sensitivity and specificity. A meta-analysis (see Bacterial Meningitis) of research has shown that 90-95% sensitivity of the Pap smear is intrinsically associated with a specificity of only 20-35% ².

Practice Point

**Unlawful death from cervical carcinoma following a negative Pap smear may arise from clinical, not laboratory, negligence**

Thus, even if one or two out of every twenty women tested have a false negative Pap smear, large numbers will have to suffer further investigation, sometimes including a potentially hazardous cone biopsy, for false positive results.

The legal implications of this medical dilemma are far-reaching. Undoubtedly some laboratories produce substandard results, and these are the legitimate target of medical malpractice litigation. However, the degree of deviation of the reading of the Pap smear from the average acceptable technical standard is a matter for expert medical opinion.

Practice Point

Occasional disability or death resulting from the investigation of non-disease is an inevitable accompaniment of some screening procedures

The zero error standard implicit in the prevalent medicolegal climate is inevitably far from medical reality ¹. The tragic death of a woman from carcinoma of the cervix after a normal Pap smear is not de facto evidence of laboratory medical malpractice. Nevertheless, misinterpretation or misrepresentation of the significance of a negative Pap smear by the clinician may sometimes amount to negligence.