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Smartphone-based immunoassay successfully measured salivary cortisol
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CHICAGO — Smartphone-based immunochromatographic technology can detect adrenal insufficiency and hypercortisolism and monitor physiologic variations in cortisol concentration, according to research presented at the joint meeting of the International Congress of Endocrinology and the Endocrine Society.
“The most obvious use of this test is going to be as a method to screen for Cushing’s Syndrome or Cushing’s Disease,” Joel R. L. Ehrenkranz, MD, of Intermountain Healthcare in Murray, Utah, said during his presentation.
“It could be used in conjunction with an adrenocorticotropic hormone stimulation test to evaluate patients for adrenal insufficiency, potentially be able to monitor cortisol replacement and characterize the diurnal rhythm of cortisol, and to measure physiologic variations in cortisol,” Erhrenkranz added.
The disposable quantitative salivary cortisol immunoassay strips — made of nitrocellulose membranes and attached cortisol-bovine serum albumin conjugate (test line) and goat anti-mouse immunoglobulin G antibody (control line) — provide results in 5 minutes and can be performed at the point of care.
The test works through a glass fiber element with colloidal gold labeled murine anti-cortisol antibodies and a saliva collection pad inserted at one end of the nitrocellulose membrane and a wicking sump at the other.
Read by spectrometer, the tests detected cortisol in phosphate-buffered saline in 0.1 mcg/ml increments, from 0.1 to 30 mcg/ml. During experiments, artificial saliva samples containing cortisol (concentrations 0.012-3.0 mcg/dl) were deposited on the pad.
The strip is inserted into a reader that aligns a collimating lens and light diffuser with a smartphone camera and flash, then imaged in 5 minutes.
“Because gold nanoparticles with a diameter of 70 to 100 nm have a plasmon surface resonance peak around 600 nm, a smartphone flash can illuminate and camera image the color generated by colloidal gold labeled anti-cortisol antibodies,” according to the abstract.
Through an image analysis app, control and test lines on the assay strip can be identified and the pixel density of the green color channel of the test line image quantified.
An algorithm, based on exponential curve and reference salivary cortisol values, converts pixel density to a cortisol value; the R value of this curve was 0.996 for salivary cortisol in the range of 0.012-3.0 mcg/dl.
“This is a competitive assay, which means the color intensity of test line is in proportion to cortisol concentration — so the brighter the line, the lower the salivary cortisol,” Ehrenkranz explained in the presentation.
Ehrenkranz pointed out two areas of value, including obviating the need to presumptively treat patients for adrenal insufficiency and making cortisol assays available to resource-limited environments.
Clinical trials have not been conducted to date. However, Ehrenkranz said he is confident the test will “clearly be able to achieve a detection level” that meets the salivary cortisol cut-off to distinguish Cushing’s from non-Cushing’s. — by Allegra Tiver
For More Information: Ehrenkranz JRL. Abstract OR48-2. Presented at: The joint meeting of the International Congress of Endocrinology and the Endocrine Society; June 21-24, 2014; Chicago.
Disclosures: Ehrenkranz reports no relevant financial disclosures.
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Jeremy W Tomlinson, BA, BM, BCH, FRCP, PhD
They’ve designed a novel assay that you can read using a smartphone, and the idea is that it’s very accessible to a patient. You can take a saliva sample, run it in the assay on the little card and read that using a smartphone. It allows you to take a measurement in a patient then-and-there and get a reading.
That would allow two things. It may well help to diagnose patients with Cushing’s, potentially. But diagnosing Cushing’s is not necessarily something you need to do at that moment; whether it would be beneficial in that regard, we have to wait and see.
Where it may well be helpful is to determine whether patients have got sufficient adrenal reserve and help make a decision on whether to instigate glucocorticoid replacement or not. If they’re acutely unwell and there’s any doubt they could be glucocorticoid deficient, you give them a steroid. This may allow you to decide without having to wait a day or two for a test to come back.
There are potentially one or two issues in terms of making sure the assay is both sensitive and specific enough. Cost is another issue, although they appear to say it’s going to be relatively inexpensive.
Another thing that’s important is we are moving a lot from measuring salivary cortisol using immunoassay to using mass spectroscopy techniques. It’s making sure the results are comparable in terms of sensitivity and detection.
It’s a great innovation, but it’s still an assay in development. There’s still a long way to go until we can actually do that at the bedside.
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