How do I construct a HealthKit HKUnit for mmol/L (millimoles per liter) for Blood Glucose values?

Question

Blood glucose values were added back in Health in iOS 8.2: https://support.apple.com/en-us/HT203113

How do I construct a HealthKit HKUnit for mmol/L (millimoles per liter) for Blood Glucose values?

The following both throw exceptions: Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Unable to parse factorization string...

HKUnit *mmolPerL = [HKUnit unitFromString:@"mmol<molar mass>/L"];
HKUnit *mmolPerL = [HKUnit unitFromString:@"mmol/L"];

Beatty

Answer 1

Can confirm that the examples provided don't work (every permutation), the alternative mg/dL unit does.

To sum up, both proposed approaches work, with similar (with the approach proposed by @cbartel the constant is actually rounded, not with the other) results regarding to the structure of the resulting HKUnit:

Printing description of mmolPerL->_baseUnits->_factors:
NSMapTable {
[6] mmol<180.15588> -> 1
[7] L -> -1
}

I'd use the shorter form using the provided constant:

HKUnit *mmolPerL = [HKUnit unitFromString:[NSString stringWithFormat:@"mmol<%f>/L",HKUnitMolarMassBloodGlucose]];

Antimony

I couldn't get unitFromString: to work but I came up with a solution using unitDividedByUnit: See my answer above. – Beatty

Answer 2

Construct two HKUnits and then perform unit math to create the complex unit:

HKUnit *mmolPerL = [[HKUnit moleUnitWithMetricPrefix:HKMetricPrefixMilli molarMass:HKUnitMolarMassBloodGlucose] unitDividedByUnit:[HKUnit literUnit]];

Beatty

After reading @jrushing answer that looks like the way to go (using the provided constant btw) – Antimony

Answer 3

In Swift 4:

let bloodGlucoseMgDlUnit = HKUnit.gramUnit(with: .milli).unitDivided(by: HKUnit.literUnit(with: .deci))
let bloodGlucoseMMolLUnit = HKUnit.moleUnit(with: .milli, molarMass: HKUnitMolarMassBloodGlucose).unitDivided(by: HKUnit.liter())

let mgdlValue = sample.quantity.doubleValue(for: bloodGlucoseMgDlUnit)
let mmollValue = sample.quantity.doubleValue(for: bloodGlucoseMMolLUnit)

Example:

81.0 mg/dL, 4.4961063718806 mmol/L
83.0 mg/dL, 4.6071213440258 mmol/L

Trillion

Answer 4

Here's an example in Swift 4:

let mass = HKUnitMolarMassBloodGlucose
let mmolPerLiter = HKUnit.moleUnit(with: .milli, molarMass: mass).unitDivided(by: .liter())

Soubrette

Answer 5

The "molar mass" passed to HKUnit's unitFromString: needs to be a decimal value representing the molar mass of the given substance.

Blood glucose has a molar mass of ~180.156 (see HKUnitMolarMassBloodGlucose in HKUnit.h for a more precise value). To construct this using unit strings you would want to use:

HKUnit *mmolPerL = [HKUnit unitFromString:@"mmol<180.156>/L"];

Unfold

Saw the constant but the documentation was not that clear imho, this is the way to go. – Antimony