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.

References

  1. Veivers v Connolly (1994) Aust Torts Reports 81-308.
  2. Morton v Jools (SC(NSW), No 10944/90, Carruthers J, 28February 1992).
  3. Hakama M, Miller AB, Day NE. Screening for cancer of the uterine cervix. Lyon: International Agency for Research on Cancer, 1986:207.
  4. Tabar L, Faberberg G, Day NE, Holmbery I. What is the optimum interval between mammographic screening examinations? An analysis based on the latest results of the Swedish two-country breast cancer screening trial. Br J Cancer 1987;55:547-51.
  5. Wood v Queensland Medical Laboratory (SC(Qld), No 116/94, Cullinane J, 16 December 1994).
  6. O'Shea v Sullivan and Macquarie Pathology Services Pty Ltd (SC(NSW) No 16943/91, Smart J, 6 May 1994); Sullivan v Micallef; Macquarie Pathology Services Pty Ltd v Micallef (CA(NSW) Nos 40289/94, 40313/94, 16943/91, Mahoney AP, Clark and Powell JJA, 16 August 1994); Macquarie Pathology Services Pty Ltd v Sullivan; Sullivan v Macquarie Pathology Services Pty Ltd (CA(NSW) Nos 40313/94, 40289/94, Kirby P, Mahoney and Clark JJA, 28March 1995).
  7. CES v Superclinics (Australia) Pty Ltd (SC(NSW) No 14479/88, Newman J, 18 April 1994); CES v Superclinics (Australia) Pty Ltd (CA(NSW) No 40266/94, Kirby P, Priestley and Meagher JJA, 22 September 1995).
  8. The plaintiff's claim for damages was unsuccessful at first instance. This was because the trial judge had accepted the defendant's evidence that the plaintiff's pregnancy did not involve a serious danger to her mental or physical health, and thus concluded that if she had had the opportunity to undergo an abortion, she would have committed a criminal offence under s82 Crimes Act 1900 (NSW). The trial judge's finding was overturned by the Court of Appeal (2-1), which ordered that the case be returned to the Common Law Division for retrial as to damages. The case was then listed in the High Court List for hearing in 1996. The Catholic Church successfully applied (3-3) on 11 September 1996 to be admitted as amicus curiae(friend of the court) in relation to the High Court appeal. As at the date of writing, the hearing has been set to resume in the High Court on 11 November 1996.
  9. Machesney v Bruni No 73-1477, 1995, Westlaw 688937 (D.D.C. June 21, 1995); Lowder v Economic Development Family Centre No 93-16747-CA (Fla, Dade County Cir Ct, September 14, 1994).