Non-invasive monitoring of diabetes through analysis of the exhaled breath condensate (aerosol)
Graphical abstract
Introduction
According to the World Health Organization, 347 million people (≈ 5% of the world's population) suffer from diabetes. Predicted to become the seventh leading cause of death, diabetes is dangerous because of its complications: cardiovascular diseases, blindness, risk of amputation, kidney failure etc. Glucose concentration in blood is the key parameter for diabetic patients: maintaining it at an appropriate level allows these complications to be postponed.
Non-invasive methods, which exclude not only injury to blood vessels, but also damage to the skin surface, are preferred for diagnostics: such methods are painless and avoid potential infection and trauma to patients. However, despite continuing efforts, the problem of non-invasive evaluation of blood glucose concentration has not yet been solved. The most promising approach appeared to be detection of blood glucose through the skin using near-IR spectroscopy. However, this method did not achieve the required sensitivity [1], [2], [3], [4]. Transcutaneous glucose delivery, for example by ‘iontophoresis’ [5], [6] has not been successful in commercialized devices.
Except for detection of glucose, it is possible to monitor diabetes on the basis of secondary metabolites. In particular, the detection of exhaled acetone has been considered for this purpose. However, despite a number of existing analyzers, the question of how the breath acetone measurement is related to the blood glucose level remains to be answered [7].
Exhaled breath condensate (EBC) is already known as an excreted liquid with high diagnostic potential [8], [9], [10], [11]. It is sampled by condensation of breath aerosol, which is formed through respiratory fluid film or bubbles bursting during opening of the bronchioles [10]. The airway lining fluid metabolite concentrations are dependent on their content in the blood [8], [10].
Unfortunately, breath condensate has not been considered an appropriate candidate for monitoring diabetes due to the reported small levels of glucose content in EBC (0.1–0.2 μM) [12], [13]. By comparison, despite the lactate concentration in blood (0.5–2 mM) being lower than that of blood glucose (> 4 mM), the lactate content in EBC has been found to be two orders of magnitude higher (≈ 0.02 mM [14], [15]).
Another contradiction has been found when considering the dilution rate (ratio of substance content in blood to its concentration in EBC) of inorganic ions. The latter (particularly sodium and chloride ions) can be employed as an ‘internal standard’ for excreted liquids due to their concentration in blood remaining nearly constant over a short time scale. The concentrations of Na+, K+, and Cl¯ in breath condensate have been reported to be a hundred [16] to a thousand [17], [18] times lower than in blood. We note the correspondence of breath condensate dilution rates for inorganic ions and lactate (100–400, above). On the basis of the reported EBC dilution rates for both lactate and inorganic ions, one would expect glucose concentration in exhaled breath condensate to be above 4–5 μM.
We report here that breath condensate actually contains glucose at the level of 0.01 mM for healthy human subjects. We have found the reason for the previously reported low glucose content: it is glucose assimilation (metabolizing) in the conventionally collected EBC. Most importantly, we demonstrate that the breath condensate glucose levels determined by our method correlate positively with blood glucose levels, thus offering the prospect of a non-invasive approach to the monitoring of diabetes.
Section snippets
Experimental
Informed consent was obtained from all subjects (healthy human volunteers from 20 to 25 years old). EBC samples were collected using a commercially available condenser ECoScreen® (Erich Jaeger GmbH, Germany) during the morning, as recommended by the ATS/ERS Task Force [19]. All work was carried out in accordance with GCP regulations. All experimental protocols were approved by the Ethical Committee of Pulmonology Research Institute (Moscow).
Experiments were carried out with Millipore Milli-Q
Results and discussion
Glucose concentration in EBC was detected using the enzyme glucose oxidase. Hydrogen peroxide (H2O2), the product of the enzyme reaction, was monitored using a sensor based on nano-scaled films of Prussian Blue, the best electrocatalyst for H2O2 reduction. As we have already reported, Prussian Blue is three orders of magnitude more active and selective than the conventionally used platinum [20], [21], [22]. In order to subtract the background caused by presence of H2O2 in EBC [24], [25], [26],
Conclusion
We conclude that an increase in blood glucose causes a corresponding increase in breath condensate glucose. A positive correlation between the variation rates of glucose content in breath condensate and in blood has been observed, offering the prospect of a non-invasive approach to the monitoring of diabetes. Indeed, with occasional blood testing for calibration, the glucose concentration in blood could be evaluated by monitoring the glucose content in the EBC.
Breath condensate collection
Acknowledgements
The authors thank Prof. R.G. Compton (Oxford University) and Dr. E. Yu. Pinchukova (BP) for corrections and fruitful comments. Financial support through Russian Science Foundation grant No. 16-13-00010 is gratefully acknowledged.
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