David McNally v Gentoo Group Limited

In a disputed medical diagnosis, the court must determine the balance of probabilities from the totality of the available evidence. That evidence may include clinical presentation, radiology, immunohistochemistry and molecular analysis. Molecular testing should not be disregarded merely because its diagnostic role is developing or because conventional testing points towards an alternative diagnosis. Its results must be given proper weight and reconciled with the other evidence. A highly specific test result may, in an exceptional case, be outweighed by the combined force of the remaining evidence.

The claimant, a painter and decorator exposed to asbestos throughout his working life, developed a malignant pleural tumour. He was diagnosed and treated for mesothelioma, but subsequent expert review produced a differential diagnosis between epithelioid mesothelioma and metastatic non-small-cell lung carcinoma.

Breach of duty was admitted and quantum was agreed at £250,000, subject to statutory deductions. The sole issue was whether the claimant had proved, on the balance of probabilities, that he suffered from mesothelioma rather than lung cancer. The court considered conflicting expert evidence concerning immunohistochemistry, radiology and molecular analysis.

1. Outcome. The claim succeeded. The court found, on the balance of probabilities, that the claimant was suffering from mesothelioma. The agreed damages were therefore recoverable, subject to the agreed deduction of statutory benefits.
2. Approach to the evidence. The diagnosis had to be determined by reconciling the immunohistochemistry and molecular analysis and considering the totality of the available evidence. Relevant evidence included the clinical presentation, radiology, pathological findings and other investigations.
3. Molecular analysis. Molecular testing was part of the material available for the differential diagnosis. It was artificial to disregard reliable molecular results once obtained, although they had to be given proper weight as part of the collective evidence. The court rejected the criticism that molecular testing was insufficiently validated to be relied upon in this case. Its diagnostic role was developing rapidly.
4. Evaluation of the competing results. The positive Claudin-4 result, with its high specificity for lung adenocarcinoma, was an important factor but was not conclusive. The molecular findings, particularly the NF2 mutation and, to a lesser extent, CDKN2A/B and TP53, together with the pleural-based radiology, supported mesothelioma. The Calretinin and Cytokeratin results were not sufficiently certain to carry significant weight. The claimant was found to be within the small percentage of cases in which a positive Claudin-4 result did not indicate lung cancer.
5. The opinions of the treating clinicians and multidisciplinary team were given limited weight, but were consistent with the conclusion reached. Counsel were directed to draw up an appropriate order for approval.