Interpreting The Sweat Test; Units Of Conductivity; Automatic Averaging - Elitech NANODUCT 1030 User Manual

Section 3: Sweat Induction and Analysis

3.2 Interpreting the Sweat Test

Units of Conductivity

Electrical conductivity–essentially an electrical measurement–should properly be measured in siemens/cm.
However, we use conductivity to indirectly measure electrolyte concentration. Since medical professionals
are more familiar with standard chemical units (such as mmol/L) for concentration, the siemens/cm units
have not been used for conductivity values in the practice of clinical chemistry, to prevent confusion.
ELITechGroup has retained the mmol/L (equivalent NaCl) unit used by other sweat conductivity instruments
in the past. Unfortunately, this unit has also produced confusion in some quarters.
It is therefore important to define and explain the meaning of this expression. The readout, both as
displayed continuously and as the electronically averaged value is expressed in mmol/L (equivalent NaCl).
This means that the sweat sample has an electrical conductivity that is equivalent to that of an NaCl
solution of the displayed mmol/L concentration (at the same temperature). THE READINGS IN SUCH
UNITS DO NOT REPRESENT THE ACTUAL CONCENTRATION OF EITHER SODIUM OR CHLORIDE
IN THE SWEAT.
The level of electrical conductivity is a function of the molar concentration of ionized molecules in a
solution. Sweat samples are made up of sodium, potassium, and a small contribution by ammonium, as the
cation contribution. The anions balancing these are mainly chloride, with lactate and bicarbonate. Thus, the
conductivity can be seen as a measure of the total electrolyte in mmol/L.
Clinical trials have amply demonstrated that sweat total electrolyte and sweat chloride are equally effective
analyses in the diagnosis of CF. As there are other ions contributing to the conductivity other than sodium
and chloride, the mmol/L (equiv. NaCl) value of a sweat sample always exceeds the actual molar sodium
or chloride concentration as analyzed specifically. The diagnostic range is therefore different from that
established for chloride.
The electrolyte selected for calibration reference happens to be sodium chloride, but it could have been
any other salt. The chemical nature of the calibration solution is immaterial, because the reference
ranges for sweat conductivity is based upon comparison with the calibration value, and is valid whatever
electrolyte is used as a reference.
For example, if lithium nitrate had been selected as the reference salt, it may have produced a possibly
different but equally reliable and effective reference range. Conductivity values would then have
been expressed as mmol/L (equivalent LiNO3). Though such an alternative calibration option is not
recommended, it would have had the advantage that since no mention of sodium or chloride is made, the
results would not be mistakenly seen as representing actual sweat sodium or chloride levels.

Automatic Averaging

Examination of the data showing the relationship between sweat conductivity and time after attaching the
sensor, obtained on all the subjects in the original test of the system, and typically shown in Fig.1, allowed
the selection of optimal settings for the averaging circuitry. After a variable lapse of time (Period A, Fig.1)
during which the sweat is gradually filling the channel in the sensor, it reaches the second electrode,
thereby completing the conductivity cell circuit and producing a displayed conductivity reading. During
the next 3 minutes (Period B, Fig.1), this reading usually falls sharply, and then assumes a steady rate of
decrease that is maintained thereafter.
This initial rapid change has been termed the "first sample phenomenon" and the reason for it is not yet
clear. In the steady phase of decrease of conductivity, the average rate of decrease is about 15% per 10
minutes (during the period 10 to 20 minutes after the first reading). The best time period for averaging
commences after the initial rapid fall stabilizes, that is at 3 minutes from the first reading, thus avoiding the
"first sample phenomenon."
It then continues for the next five minutes (Period C, Fig 1) during which the sweating rate is still near-
maximal.
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