The Ratio datatype should only be used to express a relationship of two numbers if the relationship cannot be suitably expressed using a Quantity and a common unit. Where the denominator value is known to be fixed to “1”, Quantity should be used instead of Ratio. Numerator and denominator Shall both be present, or both are absent. If both are absent, there shall be some extension present.
It includes examples where a Quantity is typically used are rates, densities, concentrations. Examples where a Ratio is used are: titers (e.g. 1:128); concentration ratios where the denominator is significant (e.g. 5mg/10mL); observed frequencies (e.g. 2 repetitions/8 hr), and where the numerator or denominator is an amount of a currency (no Unified Code for Units of Measure (UCUM) code for $ etc.). Common factors in the numerator and denominator are not automatically cancelled out. Ratios are not simply “structured numbers” – for example, blood pressure measurements (e.g. “120/60”) are not ratios.
In Ratio.extension, extensions are the additional content defined by impleentations and are always sliced by (at least) url. It may be used to represent additional information that is not part of the basic definition of the element. To make the use of extensions safe and manageable, there is a strict set of governance applied to the definition and use of extensions. Though any implementer can define an extension, there is a set of requirements that shall be met as part of the definition of the extension. There can be no stigma associated with the use of extensions by any application, project, or standard – regardless of the institution or jurisdiction that uses or defines the extensions. The use of extensions is what allows the Fast Healthcare Interoperability Resources (FHIR) specification to retain a core level of simplicity for everyone.
Ratio is used in the following places: