[The use of immunohistochemical slides for performing FISH as a useful method of conserving tissue samples]. / Ré-utilisation de lames d'immunohistochimie pour la réalisation de FISH : une solution pertinente d'épargne tissulaire.

Diagnostic updates, an increased precision of tumor sub-type classification and the development of new diagnostic biomarkers (immunohistochemistry (IHC), Fluorescence in situ hybridization (FISH) and other molecular pathology techniques), have a significant impact on pathologists' management of tissue samples. The objective of this work was to test and validate the FISH technique on detached IHC slides. An IHC technique was first performed on 30 tissue samples. After detachment of the lamella, a FISH technique was then performed according to the usual protocol with a centromeric probe. A validation cohort (n=10) with duplicate testing using a traditional FISH technique and an IHC slide with a detached lamella was then carried out. Finally, a cohort of 20 "old" cases (IHC carried out over 2years ago) was also tested. Different types of probes (specific locus, break apart) have been used. All the slides were interpreted by a technician and a pathologist. Evaluation criteria were: the general interpretability of the slide ; the percentage of labeled nuclei; intensity of the signal and the presence or absence of autofluorescence. FISH was interpretable in 100% of recently treated cases and 90% of "old" cases with a satisfactory intensity and a high percentage of labeled nuclei, without autofluorescence. The results of our study show that the reuse of IHC slides for performing FISH is a powerful means of economizing tissue samples, especially for small samples and in the absence of archived representative material.

[Analysis of interference criteria for the validation of a method file: HPS staining after fixation with zinc-formalin in comparison to fixation with the classical 4% buffered formalin solution]. / Étude de l'item d'interférence dans un dossier de validation de méthode : cas de la coloration HPS après fixation en formol-zinc et comparaison à la fixation en formol tamponné à 4.

INTRODUCTION: The department of neuropathology of Sainte-Anne Hospital uses zinc-formalin as the fixative agent for its samples. No publication referenced in Pubmed has proven the validity of this fixative agent. In the context of the accreditation of our standard staining (HPS for Hemalun-Phloxin-Saffron), we started a file for the validation of this method in which the fixative agent constitutes an « interfering ¼ substance which can modify the quality of the technique. The aim of this study was to prove that the use of zinc-formalin as a fixative agent is as suitable as the fixation with 4 % buffered formalin. MATERIALS AND METHODS: A cohort of samples fixed by zinc-formalin and by 4 % buffered formalin was performed on fresh samples, then cut and stained by HPS. The slides were interpreted by three pathologists (one of them was outside our centre)  ``blind '' to the fixative agent and they evaluated four criteria (general quality of the staining, components of the extracellular matrix, cytoplasmic details, and nuclear details) and scored them (from 0 to 3) according to the Association française en assurance qualité (AFAQAP) recommendations. RESULTS: The cohort included 43 samples. The results of the analysis showed that for samples fixed by zinc-formalin, three of the four criteria obtained significantly a better score than the samples fixed by classical formalin. DISCUSSION AND CONCLUSIONS: Our results show that the zinc-formalin fixative does not constitute an  ``interfering '' agent for the quality of the HPS staining for neuropathological samples.