Although frozen sections are a useful and accurate modality for the evaluation of tissue, there are known pitfalls. The ability to correctly interpret a frozen section will vary with the quality of the preparation. There can be considerable variation in the quality of frozen sections preparations depending on techniques employed and the training and experience of cryotomists.
Techniques used to embed the tissue may lead to considerable variability in the final "footprint" of the tissue to be compared in the frozen section and permanent section preparation. When embedding a core or wedge biopsy sample the first goal is to embed the tissue in a single flat plane. One needs to complete trim the block to achieve the complete tissue face on the frozen section while minimizing the tissue wastage. This will offer the best chance to achieve a similar size and shape tissue face (footprint) so for final comparison of the frozen section and permanent section preparation.
Face up embedding of a wedge biopsy which is more pyramidal and irregular in its dimension is more likely to result in preparations which no longer appear similar on comparison. Likewise a core biopsy not embedded in a flat plane with minimal trimming will often result in significant tissue wastage. The footprint will be different and likely smaller when to compare in the frozen section and permanent preparations. Optimal embedding can best be accomplished using a face down embedding technique as opposed to face up embedding. Face up embedding refers to simply freezing the tissue with the desired face up on a chuck with or without compression by a heat extractor. This can lead to variability in the flatness of the plane of the tissue as well as variation in "x-y" orientation of the preparation and potential tissue wastage. Face down embedding can be accomplished using a variety of techniques utilizing well bars, plastic molds or simply freeing the tissue first in a flat plane on any freezing temperature surface such as the cryostat stage or a heat extractor [9
The technique used to freeze the tissue can also introduce variability in the appearance of the frozen section preparation. Techniques which freeze the tissue more slowly will introduce a greater degree of freeze artifact which can appear as clear spaces occupied by ice crystals. The clear spaces occupied by ice crystal can resemble fat vacuoles leading to misinterpretation of the percentage of cells containing macrovesicular fat. This variation can be minimized using techniques which accomplish more rapid freezing [9
Subjectivity and pathologist experience are sources of variability. We sought to evaluate the accuracy of frozen section analysis of percent macrovesicular steatosis in assisting decision making for liver transplantation. Our institution has a busy transplantation service and hence the pathologists have experience in evaluating these specimens. This may not be the case in institutions that do not have active transplant services.
There are only a few studies that that have looked at the experience of other institutions. In a 1993 study, Markin et al.
noted that after implementation of frozen-section analysis, primary nonfunction in recipients dropped from 8.5% to 1.4% [1
]. They stated that frozen section is a reliable tool for evaluating the appropriateness of a liver for use in transplant. Fiorentino et al.
, using a cutoff of 30%, recorded an overestimation of the amount of steatosis at frozen-section in just 1.4% of biopsies. This is markedly lower than in our study. They state that frozen-section histological analysis is an effective and predictive method for liver transplantation [3
]. However, El-Badry et al.
has argued that the frozen section is no longer the gold standard for hepatic steatosis [8
]. They compared 46 cases among 4 expert pathologists from multiple institutions across Europe and the United States. The pathologists disagreed with one another, and disagreed with computerized quantification of steatosis.
When a pathologist simply "eyeballs" a slide to appraise some ratio or percentage of hepatocytes containing fat vacuoles versus those without vacuoles it is usually some form of estimate based on the pathologist's personal perception. A more objective approach would be to use a comparison chart which illustrates liver tissue with fatty change of known percentages in increments varying by 10% as defined by morphometric analysis.
Frozen-section histopathological analysis is a valuable adjunct but is not entirely accurate. Our study demonstrated that in 7% of cases, the decision to transplant might have been adversely affected by frozen section analysis. Performance of these frozen-sections in institutions where the pathologists are experienced in their interpretation, as well as meticulous technique of slide production will minimize the discrepancies. Creation of a comparison chart with morphometrically defined percentages would offer the reviewing pathologist a tool for more objective assessment of percentage of steatosis. Clinicians need to be aware that accuracy of frozen section analysis of macrovesicular steatosis in liver is good in experienced hands but some discrepancies will occur.