Personal Health Device Implementation Guide, published by HL7 International / Health Care Devices. This guide is not an authorized publication; it is the continuous build for version 2.0.0-ballot2 built by the FHIR (HL7® FHIR® Standard) CI Build. This version is based on the current content of https://github.com/HL7/phd/ and changes regularly. See the Directory of published versions

IEEE 11073 FLOATs and SFLOATs

Page standards status: Informative

IEEE 11073 uses ASN.1 MDER (Medical Device Encoding Rules) to encode floating point numbers in FLOAT or SFLOAT structures. The primary reason for this encoding is to indicate the precision of the measurement. The FLOAT is 32-bits and the SFLOAT is 16-bits. In the FLOAT, the most significant byte is the exponent and the remaining 24 bits the mantissa. Both the exponent and mantissa are signed. In the SFLOAT, the most significant 4 bits is the exponent and the remaining 12 bits the mantissa.

The exponent gives the precision. It indicates where the decimal point goes in the mantissa. A negative exponent of -N moves the decimal point to the left N places and a positive exponent of +N moves the decimal point to the right N places.

Thus, it is possible in this encoding to distinguish between the value 2, 2.0, 2.00, etc. Numerically, they all have the same value, but 2.00 indicates that the value is two but taken with a sensor that has a precision to the hundredths.

The Bluetooth GHS specifications only use FLOATs to encode quantitative values; the IEEE 11073-20601 specification uses both FLOATs and SFLOATs.

FLOAT Encoding

The table below gives some examples of FLOAT values and how they shall be encoded into the FHIR valueQuantity.value:

FLOAT Hex	FLOAT decimal	exponent	mantissa	FHIR encoding
0x00000002	2	0	2	2
0xFF000014	4278190100	-1	20	2.0
0xFE0000C8	4261413064	-2	200	2.00
0x01000002	4098	1	2	20
0x02000002	33554434	2	2	200
0x000000C8	200	0	200	200
0x000004D2	1234	0	1234	1234
0x00FFFB2E	16775982	0	-1234	-1234

SFLOAT Encoding

In the SFLOAT case the above values would be encoded as follows:

SFLOAT Hex	SFLOAT decimal	exponent	mantissa	FHIR encoding
0x0002	2	0	2	2
0xF014	61460	-1	20	2.0
0xE0C8	57544	-2	200	2.00
0x1002	4098	1	2	20
0x2002	8194	2	2	200
0x00C8	200	0	200	200
0x04D2	1234	0	1234	1234
0x0B2E	2862	0	-1234	-1234

Special FLOAT and SFLOAT Values

The special values are used to indicate that the sensor was unable to acquire a valid measurement. The Observation.dataAbsentReason element is used to indicate the meaning of the special value. The Observation.value[x] element is not populated when a special value is reported.

FLOAT Hex	SFLOAT Hex	Meaning	FHIR Mapping
0x007FFFFF	0x7FF	Not a Number (NaN)	not-a-number
0x007FFFFE	0x7FE	Positive Infinity (+Inf)	positive-infinity
0x00800002	0x802	Negative Infinity (-Inf)	negative-infinity
0x00800000	0x800	Not at this resolution (NRes)	error
0x00800001	0x801	Reserved for future use (RFU)	error

Explanation of Special Values:

• NaN (Not a Number): Indicates an invalid result from the measurement process used by the PHD.
• NRes (Not at this resolution): Indicates that the value cannot be represented with the supported range and resolution of the PHD. For example, a body thermometer supporting values between 30°C and 45°C may report NRes if used in freezing or boiling water.