Introduction
The screening, testing and pathology services of laboratories are important adjuncts to clinical practice. Indeed, failure to utilise such services properly can be the basis for a claim of medical negligence. Examples include failure to carry out appropriate testing to determine the rubella status of a pregnant patient1, and failure to disclose an abnormal cervical smear result.2 However, when utilising laboratory services, medical practitioners need to be aware of the potential for both false negative and false positive results, and understand the limited role which screening tests should play in diagnosis and treatment.
False negatives
False negative results of certain screening tests are well recognised. For example, the smears used to screen for cervical cancer have a 10% false negative rate3, and a 20% false negative rate has been reported for mammographic screening for asymptomatic breast cancer.4
Mere communication of a false negative result to a patient does not of itself amount to medical negligence. This is illustrated by litigation commenced against testing laboratories5 e.g. where the laboratory has been negligent in its pathology testing of a specimen removed from an otherwise asymptomatic patient. In such a case, provided the patient is asymptomatic, and provided the medical practitioner has conformed to acceptable medical practice in advising the patient to present for re-screening or re-testing at an appropriate interval, or upon development of symptoms, there is no basis for a legitimate medical negligence claim. To ensure a successful defence to such a claim, a practitioner should record his advice about re-screening or re-testing in the patient's notes, should diarise necessary follow-up consultations, and should ensure that the patient is notified of any necessary follow-up consultations.
When a medical practitioner communicates a false negative screening or test result to a patient who does have symptoms, albeit unexplained, the position is clearly different. In such a case, the practitioner's reliance on a false negative result as a justification for not carrying out further (expeditious) investigations may amount to medical negligence. This can be so even if the laboratory had itself been negligent in producing the false negative result.
The widely-publicised O'Shea6 litigation is a good example of the potential legal liability of a medical practitioner who relies on a false negative screening result as a basis for not carrying out further urgent investigation of a patient with unexplained symptoms. In this litigation, the court found that the laboratory had been negligent in misinterpreting the patient's cervical smear. However, the court also found that the medical practitioner who had commissioned the test had been equally negligent in assuming, in view of evidence accepted by the court that the possibility of a false negative rate could be as high as 20%, that her patient's result would be accurately indicative of cancer, one way or the other. The court rejected the practitioner's argument that the laboratory's negligence had 'eclipsed' her own negligence in failing to make a proper investigation or referral when first informed by her patient of postcoital bleeding, and emphasised that the practitioner's negligence was independent of, and both prior to and subsequent to that of the laboratory. The court also emphasised that screening is not a proper tool for diagnosis, and that screening should play only a minor role in patient diagnosis and treatment.
The same potential for a successful medical negligence claim is illustrated by litigation7 brought against a clinic and several medical practitioners employed by the clinic. They each failed to diagnose the pregnancy of a sexually-active young patient who had presented on 5 occasions with amenorrhoea, and who had repeatedly expressed her wish to terminate any pregnancy. The patient was not given a blood test for pregnancy until her second consultation, at which stage she was 6-7 weeks pregnant. Relying upon the false negative result of that test, a further blood test and physical examination were not carried out until the fifth consultation. The patient's pregnancy was finally diagnosed and conveyed to her (by a medical practitioner not employed by the clinic) at 19-20 weeks.8
False positives
Certain laboratory diagnostic tests, such as the antibody ELISA test for hepatitis C and HIV, have a high false positive rate. Just as failure of diagnosis through inappropriate reliance on false negative results is a clear basis for medical negligence claims, so too is misdiagnosis through inappropriate reliance on false positives.9 Misdiagnosis can result in substantial emotional harm to the patient, as well as physical harm e.g. if a patient who supposedly is HIV-positive is prescribed zidovudine and suffers adverse effects such as severe anaemia, pancytopenia, myopathy, or liver damage. Primary liability, if any, logically should lie with the laboratory which produced the false positive result, but clearly the medical practitioner who commissioned the test and who communicated its false positive result may also attract some liability for failure to ensure that appropriate confirmatory tests were carried out.
