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BackgroundNumeric results of pathology tests are sometimes returned as a range rather than a precise value, e.g. "<10". In health data research, test result values above or below clinical threshold values are often used to categorise patients into groups; however comparators ( etc) are typically stored separately to the numeric values and often ignored, but may influence interpretation.MethodsWith the approval of NHS England we used routine clinical data from 24 million patients in OpenSAFELY to identify pathology tests with comparators commonly attached to result values. For each test we report: the proportion returned with comparators present, split by comparator type and geographic region; the specific numeric result values returned with comparators, and the associated reference limits.ResultsWe identified 11 common test codes where at least one in four results had comparators. Three codes related to glomerular filtration rate (GFR) tests/calculations, with 31-45% of results returned with "≥" comparators. At least 90% of tests with numeric values 60 and 90 represented ranges (≥60 and ≥90 respectively) rather than exact values. The other tests - four blood tests (Nucleated red blood cell count, Plasma C reactive protein, Tissue transglutaminase immunoglobulin A, and Rheumatoid factor), two urine tests (albumin/microalbumin) and two faecal tests (calprotectin and quantitative faecal immunochemical test) - were returned with "≤" comparators (29-86%).ConclusionsComparators appear commonly in certain pathology tests in electronic health records. For most common affected tests, we expect there to be minimal implications for researchers for most use-cases. However, care should be taken around whether results falling exactly on clinical threshold values should be considered "normal" or "abnormal". Results from GFR tests/calculations cannot reliably distinguish between mild kidney disease (60-<90) versus healthy kidney function (90+). More broadly, health data researchers using numeric test result values should consider the impact of comparators.

Original publication




Journal article


Wellcome open research

Publication Date





Nuffield Department of Primary Care Health Sciences, University of Oxford, Oxford, England, OX2 6GG, UK.


OpenSAFELY Collaborative