Appendix E Theory Of Operation - Elitech ChloroChek Chloridometer 3400 User Manual

APPENDIX E
THEORY OF OPERATION
INTENDED USE
The ChloroChek Chloridometer test system is intended for the quantitative in vitro diagnostic determination of
chloride in human sweat using the principle of coulometric titration. Sweat chloride measurements are used in
the diagnosis of Cystic Fibrosis. It is for use in Clinical Laboratory settings. The ChloroChek Chloridometer test
system consists of the ChloroChek Chloridometer and the ChloroChek Reagent Set.
The ChloroChek Reagent Set (SS-248), is to be used on the ChloroChek. It is used as the titration matrix during the
titration process.
The 100 mmol/L NaCl/H O Standard Solution (SS-251), is to be used on the ChloroChek. It is used as a calibration
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verifier, and quality control solution.
ELITech Group Sweat Controls (SS-150), levels #1, #2, and #3, are to be used on the ChloroChek. They are used as
quality control solutions.
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CLINICAL SIGNIFICANCE
The quantitative measurement of the chloride in sweat (commonly called the "sweat test") is used to confirm the
diagnosis of cystic fibrosis (CF). With an approximate incidence of 1:3200 in Western Europe and the USA, CF is
the most common life-threatening genetic disease within the Caucasian population. It is an autosomal recessive
disorder characterized by viscous secretions that affect the exocrine glands, primarily in the lungs and pancreas.
Patients with CF have an increased concentration of sodium, chloride, and potassium in their sweat. The criteria
for the diagnosis of CF include the presence of one or more characteristic phenotypic features, a history of CF in a
sibling, or a positive newborn screening result; and an increased sweat chloride concentration by pilocarpine
iontophoresis on two or more occasions, or identification of two CF-causing mutations or demonstration of
abnormal nasal epithelial ion transport.
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METHOD
Based on CLSI recommendations. C34-A3 (2009): Coulometric titration.
PRINCIPLE
ChloroChek operates according to the principle of coulometric titration. Two silver electrodes—the generator
electrodes (anode and cathode) –are dipped into a measuring vessel filled with working solution. The working
solution consists of a buffer and a colloid stabilizer that keeps the silver chloride, which arises later on, in
suspension.
Since the buffer does not contain any silver ions, the silver ion concentration, and thus the indicator current (see
below) is brought to a specific end point. By means of a constant current (generator current) between the two
silver electrodes, a constant amount of silver ions is released at the anode. The silver ion concentration is
maintained by the measurement electrodes (indicator electrodes), which are dipped into the solution. By adding
a chloride sample, the free silver ions form a non-soluble silver chloride precipitate together with the free
chloride ions of the sample.
→ AgCl
+ –
Ag + CI
The indicator current drops, and by controlling the generator current, silver ions are released until all chloride
ions are precipitated as silver chloride. This restores the original silver ion concentration (end point).
The period of flow of the generator current is measured during the titration process and is proportional to the
chloride ion concentration.
Depending on the type of sample, no more than 50 measurements should be carried out using one batch of
working solution.