Introduction

HCL Pap reagent is a differentiation reagent used primarily in regressive staining of cytology samples according to Papanicolau. It contains very low concentration of hydrochloric acid and it enables excellent differentiation of nuclei and non-nuclear structures. After the samples are stained with hematoxylin during regressive staining method, the residual dye is washed of from the nucleus, as well as off the cytoplasm. That leaves cells with clearly defined cellular details and intercellular structures to be further stained using OG-6 (or Orange II) and EA reagents.