Discordance between CO2- and pH-Based Apnea Test Criteria for Brain Death: The Importance of the Pre-Test Acid–Base State

Background and Objectives: Apnea test criteria differ between countries and have changed within them. The 2010 American Academy of Neurology guideline accepted a PaCO2 ≥60 mmHg or a rise of ≥20 mmHg above baseline; the 2023 AAN/AAP/CNS/SCCM consensus guideline requires both together with an arterial pH <7.30; and the Turkish national standard, restated in December 2025, requires both carbon dioxide conditions but no pH target. We examined how the same real-world apnea tests are classified under these three criteria, and what accounts for any disagreement between them. Materials and Methods: All three criteria were applied to every apnea test performed during brain death determination in a tertiary intensive care service between 2019 and 2026 (n = 331). The primary endpoint was the 2023 consensus criterion. Determinants were modelled with Firth penalised logistic regression, with the Turkish criterion as a negative control. Results: 237 adults were evaluable under all three criteria. The proportion of tests meeting the criterion fell from 98.3% (AAN 2010) to 87.8% (Türkiye 2025) to 78.1% (2023 consensus); no test gained positivity at any step, and 48/237 (20.3%) were positive under at least one criterion but not all three. All 23 tests that met the Turkish criterion but failed the 2023 criterion had begun with a pre-test pH above 7.45. Among the 118 tests begun within the pH window that guidelines require, the two criteria were met by the same patients (106/118 (89.8) each). Pre-test bicarbonate was associated with meeting the 2023 criterion (OR 0.84 per mmol/L, 95% CI 0.78–0.92) but not with the Turkish criterion (OR 1.09, 95% CI 0.98–1.20). Conclusions: In this cohort, disagreement between contemporary apnea test criteria was determined mainly by whether the pre-test acid–base requirement had been met, rather than by the criteria themselves.