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Poster 18
04149 Three years later: how the previously defined visual threshold of the Hoesch test works
  1. Galina Zemtsovskaja1,2,
  2. Siim Iskül1,2,
  3. Küllike Palk2,3
  1. 1Clinical Chemistry and Haematology Department of the Laboratory, North Estonia Medical Centre Foundation, Tallinn, Estonia
  2. 2Estonian Porphyria Centre, Tallinn, Estonia
  3. 3Gastroenterology Centre of the Internal Medicine Clinic, North Estonia Medical Centre Foundation

Abstract

Background Hoesch test is a rapid test based on porphobilinogen (PBG) reaction with Erlich reagent. The method is available 24/7 in our hospital due to its quite simple measurement protocol. The rapid test has several limitations caused by difficulties in visual assessment of the resulting coloured solution, as well as interferences that cause false positive or false negative results. We have previously validated the Hoesch test in our laboratory conditions and determined the threshold value to be around 20.8 µmol/L. In our hospital, the trueness of the rapid test results are usually confirmed as soon as possible by the PBG measurement on commercial solid phase extraction (SPE) method separating with an anion-exchange column, treatment Ehrlich’s reagent and photometric measurement.

The aim of our investigation was to evaluate the actual performance of the previously found threshold value of the Hoesch test in a routine working process.

Methods We analysed the qualitative results of all Hoesch tests performed during recent 3 years together with the corresponding results of the quantitative SPE measurement. Urine PBG (U-PBG) values measured by the SPE method were divided into positive and negative groups based on the results of the Hoesch test. Receiver operating characteristic (ROC) curve analysis was performed by a statistical program MedCalc V19.1.

Results U-PBG data (n=272) were distributed between groups with corresponding positive (n=37) and negative (n=235) Hoesch test results. ROC curve analysis showed that the optimal threshold level for U-PBG, as determined by the Youden index, was >13.6 μmol/L (sensitivity 100%, specificity 94.9%), area under the curve 0.993. Our previously established threshold value of about 20.8 µmol/L fits well between closest to optimal the ROC curve points of 19.72 µmol/L and 21.41 µmol/L with a sensitivity of 97.3 and 91.9%, a specificity of 96. 6% for both points.

Conclusion Our previously determined U-PBG threshold value of approximately 20.8 μmol/L, which corresponded to a positive Hoesch test by visual assessment, performed very well in routine working process. When compared to the optimal threshold value determined by Youden index, it demonstrated a slight loss of sensitivity but an increase in specificity. The better specificity of the threshold value is especially important in the management of this group of diseases with low prevalence.

http://creativecommons.org/licenses/by-nc/4.0/

This is an open access article distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 4.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited, appropriate credit is given, any changes made indicated, and the use is non-commercial. See: http://creativecommons.org/licenses/by-nc/4.0/.

https://doi.org/10.1136/bmjgast-2024-ICPP.47

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