The index test is the UCP-LF strip test detecting and quantifying CAA levels. LF strips can be analysed with POC care readers [47 (link)] or, as used in this study, with a multistrip benchtop reader (Upcon; Labrox Oy, Turku, Finland) capable of analysing 20 strips at a time. The UCP-LF CAA test was developed at Leiden University Medical Center (Leiden, The Netherlands) [38 (link), 48 (link)]. So far, the tests are produced in-house at LUMC in a batch-wise manner (ca. 2000 tests per batch), fully quality-controlled using standardized reference samples, ensuring specificity and sensitivity. A dry reagent format is available that can be stored and transported at ambient temperature [39 (link)]. Here, 417 μl of fresh or stored (− 20 °C) urine will be tested by the UCP-LF CAA urine test following published procedures [48 (link)] including a concentration step (centrifugal Amicon Ultra 0.5 mL filters with a 10 kD cut-off; Merck Millipore) after pre-treatment of the urine with 1/6 volume of 12% trichloroacetic acid and (UCAAhT417 format) [49 ]. All measurements will include a set of CAA-spiked standards into a negative urine to determine the threshold of positivity that is set to ≥2 pg/ml.
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