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Methodology

How Null Health builds, sources, and tests every formula on the site.

Where the math comes from

Every formula across all three calculators is grounded in a citable source - FDA prescribing information (e.g. semaglutide’s 165 h half-life from the Wegovy USPI), a USP/ISO standard (e.g. U-100 syringe graduations from USP <1601> / ISO 8537), a clinical practice guideline (e.g. the Endocrine Society 2018 TRT band), or a peer-reviewed pharmacokinetics reference (Rowland & Tozer; Gibaldi & Perrier). Each calculator’s assumptions page lists the math and the source for every assumption alongside its limitations. A cited formula speaks only to the mathematical claim - it does not imply the calculator output is appropriate for any individual person.

  • Doses per week from the calendar (7 days divided by the chosen interval).
  • Volume per injection from the standard concentration relationship volume = mass / concentration.
  • U-100 syringe units from USP <1601> and ISO 8537:2016 (100 units = 1 mL exactly).
  • Doses per vial from the integer floor of vial volume divided by per-injection volume.
  • Steady-state PK metrics (peak, trough, accumulation ratio, time-to-95% steady-state) from the one-compartment closed-form identities in Rowland & Tozer, Clinical Pharmacokinetics and Pharmacodynamics, 4th ed., chapter 6, applied to each drug’s labelled terminal half-life.
  • Per-drug pharmacokinetic constants (half-life with 90% CI, bioavailability, absorption rate constant, apparent Vd/F) from each drug’s USPI clinical pharmacology section, with phase-1/phase-2 fallbacks for investigational compounds.

How every assumption is documented

  1. 1. Source on file. Every assumption names a specific citable source - FDA prescribing information, clinical guideline, USP/ISO standard, or peer-reviewed reference.
  2. 2. Test coverage. The math is exercised by unit tests covering the documented edge cases, with tolerances tight enough to detect drift.
  3. 3. Limitations written into the page. Every assumption’s detail text spells out what the math does not model (insurance reimbursement, individual variability, clinical decision-making, etc.) alongside what it does.
  4. 4. Public review trail. Each calculator’s page records the last review date. Cited formulas speak to the mathematical claim only - never to whether the output is right for any specific person.

Per-calculator details

The full assumptions list lives on each calculator page itself:

Privacy

Calculators run entirely in your browser. Inputs are stored in local storage on your device so the form remembers what you typed; nothing is transmitted to any server, and inputs are not logged. Shared URL state (e.g. links containing ?weeklyMg=...) is purely a convenience for sharing setups - it does not get logged either.