Applications of the Conductivity
Can be applied to any chemical change where there is a change in ionic composition as a result of the reaction - this is explained in detail in the Introduction
BLOOD & URINE TESTS - which can be done using this method? - see list below
To demonstrate the sensitivity of the method: only 0.5 uL of urine or 20 uL of blood is required for a urea assay. Note this is 10x the detection limit, so 2 uL blood could be used. This is much less than a small pinprick
The method can be applied to most enzyme-catalysed reactions. The apparatus & methods presented on this site overcome the drawbacks that have prevented this method from being more widely used in the past.
Reactions that are easily measured are:
HYDROLASES [not amylase & friends]
More difficult would be DEHYDROGENASES, although there is no change in the number of ions there is a change in mobility of the ions, [e.g. from NADH to a proton], so this may well be measurable. Spectroscopy will remain the favourite method for these enzymes I think.
ISOMERASES look unsuitable for this method
The technique is easy to use and could be applied for clinical tests [with the exception of glucose, which has been 'solved with inexpensive miniaturised sensors aand readout systems - brilliant!!
The same technology could easily be applied to more generalised measurements of the enzymes described above. The main problem I forsee is the shrinking of the temperature control system which needs to be accurate for the measurement of rates of reaction. The enzyme can be added as a reaction component or, as in the glucose detectors, be attached to the electrode surface, so that only the substrate and buffer need to be added.
The apparatus as presented here is ideal for school/ college research projects & it could be commercialised using modern minitiaturisation methods for widespread use in CLINICAL TESTING, not only in a lab setting but most importantly in the FIELD, since it is easily transportable, e.g. on a bicycle.
CLINICAL TESTS FOR BLOOD USING THE CONDUCTIVITY METHOD
Creatine kinase XX
Lactate Dehydrogenase X
Alkaline phosphatase XX
Angiotensin Conv. Enzyme XX
Number of X's indicates sensitivity, assessment is based on ionic (& mobility) change upon reaction. Glucose is excluded as excellent inexpensive kit is available.
Many urine tests can also be done. One important one, creatinine cannot be done, since there is no charge difference between substrate & products.
For a comparison with a modern hand-held bedside clinical analysis system see Abbott i-STAT. An assay set takes just 2 min. This is used in some Emergency Departments for rapid diagnosis/ treatment but has not generally been adopted because of the cost, which is £10-£20 per assay set (about 6-8 tests of different parameters for each chip). The capital cost of the equipment is ca. £7000 - £10000. It typically requires about 100 microlitres of blood per test set (1 chip).
Radiometer & Siemens manufacture high throughput systems for central laboratories, that perform the testing for the whole hospital.