A Cardio-Metabolic Cascade for Personalised Haematocrit Tolerance and Dynamic Testosterone Ranges

Haematocrit elevation is the most common dose-limiting adverse effect of testosterone therapy, and clinical guidelines manage it with fixed thresholds applied equally to every patient. This note describes the cardio-metabolic cascade used in the TRT Plus app, a deterministic rule-based model that makes haematocrit tolerance and the displayed testosterone range conditional on a person's broader cardio-metabolic profile. The cascade has three coupled parts. Model A counts how many of six markers (systolic and diastolic blood pressure, apolipoprotein B or LDL cholesterol, C-reactive protein, body fat percentage and HbA1c) are above their target band and assigns a five-level tier that sets the haematocrit green and amber ceilings; a complete profile with no elevated marker extends the green ceiling from the guideline-anchored 50% to 54%. Model B shifts the testosterone target range per marker, raising its ceiling for each marker in range and lowering both ceiling and floor for each marker above range, within hard limits. Model C caps a high-normal testosterone to amber when haematocrit is red. We give the formal definitions, design rationale, worked examples, implementation conventions and limitations. The approach is deliberately less conservative than the guideline in favourable profiles and has not been validated against clinical outcomes; it is published as a methods description, not as clinical guidance.