Doctors could one day drop tears to diagnose disease by Raman spectroscopy.

Nicholas Stone and Jacob Filik at the Gloucester Royal Hospital, UK, have used a technique called drop coating deposition Raman (DCDR) spectroscopy to detect changes in protein concentrations at the microlitre levels found in human tears. As Stone explained, 'infection causes the protein composition in body fluids to fluctuate, so the ability to detect small changes in protein concentration is important for disease diagnosis.'

Fluctuations in protein levels in tears can indicate disease

Tear-drying patterns are known to differ with infection and have been used in diagnosis for some time, but DCDR takes this a step further by analysing individual proteins to pinpoint which disease is present. DCDR concentrates solutions, moving them across a substrate by capillary flow, making it easier to obtain their Raman spectra. The weak solution is continually replenished by liquid from the centre and is concentrated in a characteristic coffee-ring drying pattern as the solvent evaporates. Stone was able to use the method to detect small concentration changes in mixtures of lysozyme, lactoferrin and albumin, which together make up 95% of the proteins found in tears.

Andrew Berger an expert in biomedical optics at the University of Rochester, US, said that the 'work shows that DCDR can be a valuable tool for analytical chemists. Raman spectroscopy has great specificity, but the signals are often too weak. What's exciting is that the simple process of evaporation can be harnessed to increase the signal, shift the barrier, and open up a new range of possible applications.'

But Stone cautioned that 'there are a number of challenges to be overcome before DCDR can be reliably used for disease diagnosis. These include determining whether the technique is sensitive enough to detect the changes in fluids caused by disease and how non-proteins and contamination in the samples influence the results.' He added that there is also a 'difficulty of finding which diseases would benefit most from early detection and would also display large enough systematic protein changes to be detected.'

Janet Crombie