The Measurement of the Physical Properties of Cystic Fibrosis Sputum using Texture Analysis

D.M Trainor, S. A. Barker, D. Q. M. Craig & J. S. Elborn
Queen's University Belfast

Cystic fibrosis (CF) is the largest autosomally recessive genetic disorder in children worldwide.  The disorder results in the production of thick, viscous mucus/sputum in the lungs which is difficult to clear from the airways, reduces pulmonary function, is a focus for infection and acts as a barrier to the diffusion of inhaled drugs.  The ease of clearance of sputum by cough or ciliary action is governed by its physical properties, such as viscosity, adhesiveness and cohesiveness.

Previously used analysis techniques

To date, physical characterisation of CF sputum has predominantly utilised the technique of magnetic microrheology in which a small steel ball under electromagnetic control moves through the sample.  The motion of the ball is monitored, and rheological parameters thus inferred.  The use of this technique presumably arose due to the small volumes of sample available form very young patients.  However, there are concerns about the reliability of this technique, as the small sample volumes used (5 to 10mL) raises issues of sample preparation and selection, and any inhomogeneity in the sample selected would cause exaggerated ball displacement, thus affecting interpretation of the results.

Quantitative methods improve qualitative assessments

Samples of CF sputum may be qualitatively graded using criteria based on their appearance.  In our laboratory, we have used colour descriptors and a visual viscosity index (VI) to describe the expected viscosity of the sputum samples.  The VI ranges from 1 to 4, where 1 corresponded to an apparently very non-viscous sample and 4 corresponded to an apparently very viscous sample.  Although the VI is a useful tool in the initial assessment of the physical properties of the sputum sample, it is obviously subjective, with consequent operator dependence.  There is, therefore, a need for a more qualitative method of assessing the physical properties of sputum.

A Quantitative Assessment of Sputum Adhesivity

Using the TA.XT2 Texture Analyser Queen's University of Belfast have developed a method for the measurement of sputum adhesivity, work of adhesion and cohesiveness.  The adhesiveness is obtained by the force required to detach the probe from the test material and the cohesiveness corresponds to the distance travelled before separation of the probe and test substance occurs.  Five replicate adhesion tests have been performed on samples (10mL) of sputum tested using a 10mm diameter Perspex cylinder probe to ensure good sample contact and consistent probe insertion volume.  Results have shown values for all three textural parameters increased with increasing sputum viscosity with comparatively low standard deviation and of the same order of magnitude as would be expected for conventional rheological investigations.

Texture analysis shines a light

Patients are now reaching adulthood, due to improved therapy and nutrition, and larger sputum samples are now available, thus allowing more in-depth physical characterisation.  Sputum may be tested by the TA.XT2 as received and tested without need for pre-treatment otherwise stored refrigerated at 4°C for one week following collection, with little change in its physical properties.  This may have implications for the design of clinical studies whereby sputum samples are collected and stored prior to analysis.  Researchers at Queen's University have concluded that these results have shown that it is possible that the TA.XT2 may be developed to provide a numerical description of the consistency of CF sputum allowing assessment of disease progression and effectiveness of clinical therapy.

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