Liquid Elastase Samples To Test, or Not to Test? That is The Question… Alison Jones, Consultant Clinical Biochemist, York and Scarborough NHS Foundation Trust Author – Amanda Appleton, Senior Product Manager, Alpha Laboratories Ltd. “It is well documented that liquid/ watery stool samples should not be tested for faecal elastase due to the risk of a false low result suggesting pancreatic insufficiency (a false positive). This is not a failure of the assay – it works in exactly the same way whether the sample is formed or liquid. The challenge is more in the interpretation of the result. Is the low result due to a poorly pancreas, or is it the body’s reaction to some other pathology causing dilution of the elastase in an individual with an otherwise functioning pancreas? Although I understand the reasons, taking a hard line on samples and rejecting liquid ones isn’t overly helpful. In the main I would suggest that elastase testing comes from secondary care settings where patients are more likely to have longer term gastric issues who might never produce a more formed sample. Even if the patient does have pancreatic insufficiency, it is often associated with diarrhoea! So, is there an alternative? Alison Jones (Consultant Clinical Biochemist) from York and Scarborough NHS Foundation Trust discuss a recent audit conducted, and the recommendations that have been implemented. The reason we started the project was based on an anecdotal feeling that we were getting a lot of watery samples that weren’t being repeated despite reporting the results with comments suggesting the result may be unreliable. We retrospectively reviewed three years (2018 – 2020) of elastase results, which comprised just over 5000 tests, and 5.7% of these (288 samples) were identified as watery1. We then identified if we had received a formed sample from the same patient to try and validate the result from the liquid sample - not a technically accurate method, but the only basis we had to compare. Potential false positives There were 60 samples where we had received both a liquid and a formed stool sample within 12 months of each other. We confirmed what many people have published: Watery samples can cause a false positive result, with 71% giving a positive (<200µg/g) result with the liquid sample but a negative result (>200µg/g) from the formed sample. This means that for these patients a repeat sample is required to see if the positive result is genuine or an artifact of the composition of the sample. There were a further 87 results with values <200µg/g from liquid samples with no repeat sample received. The clinical notes from 37 of these patients were reviewed. Pancreatic Enzyme Replacement Therapy (PERT) was started in 15 patients, of which seven had surgical, imaging, histological or clinical evidence of pancreatitis, and a positive response to the PERT was noted in a further four. One patient remained symptomatic on PERT, suggesting this was an inappropriate diagnosis and treatment in this case. The potential unreliability of the faecal elastase results and/or the need for a repeat sample was only recorded in 5 of the 37 notes examined. 1 Visual flow chart of 2018-2020 review of total number of patients with watery samples 10 Perspective
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