UCUM units
One unambiguous code for every unit of measure — because mg/dL vs mmol/L confusion is a patient-safety issue, not a formatting nit.
In one line
UCUM (Unified Code for Units of Measure) defines machine-readable codes for units —
mg/dL, mmol/L, mm[Hg], kg/m2 — so software can validate, convert and compare
quantities instead of guessing from free text.
How it works
UCUM is a grammar, not a list: units compose algebraically (mg/dL, mL/min/{1.73_m2}),
case matters (mg ≠ Mg), and arbitrary units wear braces ({breaths}/min). Because
codes are parseable, a system can verify that a value's unit is commensurable with the
expected one and convert automatically — the difference between a chart that plots and
a chart that lies.
Where it shows up in digital health
Observation.valueQuantity.code in FHIR is bound to UCUM; LOINC's example units are
UCUM; device data (IEEE 11073 MDC units map to UCUM) and every cross-border lab
exchange depend on it. The cautionary tale to remember: glucose reported in mg/dL read
as mmol/L is an 18-fold error — units are clinical data, treat them as such.