IHC Technique Guide: Paraffin Block Trimming and Sectioning

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6 2 月, 2026

On 6 2 月, 2026

Preface:

For an excellent section, the ‘cutting’ step cannot be overlooked. Sectioning may seem simple, but it contains significant technical content and also tests the skill level of pathology technicians. Strictly following operational specifications while paying attention to some details is very helpful in improving the quality of sections.

In the actual slide preparation process, no matter how strictly operational specifications are followed, some minor issues always arise during the sectioning stage, such as irregular sections, skipping, too thick, too thin, curling, scratches, and fragmentation. Under the microscope, these manifest as holes or lack of staining, which not only affects the appearance of the section but, more importantly, impacts subsequent diagnosis. Paying attention to small details in sectioning helps produce more standard sections.

Preparation Before Sectioning

Knife Sharpening:Non-disposable knives should be sharpened after each use, sharpening for 10-15 minutes until the blade feels needle-like to the touch; disposable blades should be replaced when a single cutting edge has been used for more than 6-8 wax blocks or when blade marks appear on the sections.
Trimming Wax:The principle is rough trimming – fine trimming – precision trimming

Rough Trimming: The setting of the trimming depth should fully consider the size and nature of the tissue. Generally, for a standard tissue block of 2cm x 1.5cm x 0.3cm with medium texture, the rough trimming depth can be set at 20-30μm. For some small specimens, it is recommended that the rough trimming depth be less than 10μm.

Fine Trimming:Fine trim at a thickness of 4μm, with gentle movements, carefully observing whether the trimmed wax ribbons are flat and smooth.

Sectioning Tips

Do not use the same blade position for trimming and sectioning; this saves the blade and improves quality;
When using a new blade, first cut small tissues or solid tissues like lymph nodes, then cut fat and muscle tissues, and finally cut bone tissue;
When fixing the wax block, pay attention to the embedding direction, tissue layers, etc., placing difficult-to-cut parts on top, such as skin, capsule, serosa, etc.;
Operate the microtome with even force, recommended at 2 seconds per rotation.

Handling Sectioning Problems

Sections curling, sticking, or not forming continuous ribbons

Possible causes: Blade not sharp enough, incorrect blade angle, section too thick, wax block not cold enough, or dry air.

Countermeasures: Identify the cause and handle accordingly, check the blade; increase indoor humidity; chill the cutting surface of the wax block; blow gently on the ribbon while cutting; lower room temperature; adjust section thickness.
Wax ribbon curling

Possible causes: Irregular or unevenly trimmed wax block, blade not parallel to the wax block.

Countermeasures: Trim the wax block flat and completely, adjust the left-right height of the blade to ensure the bottom edge of the wax block is parallel to the blade’s cutting line.
Uneven thickness

Possible causes: Microtome carriage damage, wax between the pressure plate and carriage, loose or worn microtome parts, wax block too hard.

Countermeasures: Check and confirm the cause, clean thoroughly, replace parts; if the wax block is too hard, it can be rewarmed and moistened before sectioning.
Scratches or cracks on sections

Possible causes: Nick in the blade, impurities in the wax, calcification in the tissue, bone fragments or suture knots.

Countermeasures: Change the blade; remove foreign objects; if due to calcification or bone, place the tissue surface over 30% hydrochloric acid for several hours, then wash and recut.
Wavy appearance on sections

Possible causes: Over-dehydration during tissue processing, dull blade, angle too large, or sectioning speed too fast.

Countermeasures: If confirmed to be caused by over-dehydration, gently wipe the tissue surface with a wet gauze soaked in warm water to help alleviate the issue.
Sections fragmented or incomplete

Possible causes: Tissue issues, poor dehydration, over-dehydration/clearing, paraffin infiltration temperature too high or time too long.

Countermeasures: For tissues with poor dehydration, place the wax block in a 65°C paraffin oven overnight, then re-embed and section;For over-dehydrated tissues, freeze and moisten the wax block before cutting.
Sieve-like holes

Possible causes: Lack of fine trimming, common in special tissues like liver, tonsil, lymph node, spleen, etc.

Countermeasures: Fine trim until the surface has no white spots; for brittle tissues, rewarm appropriately and cut slowly.
Special Tissue Handling

Lymph node tissue: Before sectioning, press thumb dipped in warm water against the tissue surface for 5-6 seconds, then chill with ice for 3-4 seconds and immediately section.

Fatty tissue:Before sectioningChill with ice for 5-10 seconds and immediately section to obtain intact tissue.

Tissue with many blood clots: Before sectioning, press thumb dipped in warm water against the tissue surface for a few seconds and immediately section to reduce fragmentation.

Brain tissue: Sections tend to stick to the blade and wrinkle. Before sectioning, press thumb dipped in warm water against the tissue surface for 5-6 seconds, then chill with ice for 3-4 seconds and immediately section.

Although slide preparation is a fundamental task, the sectioning step, though small, is of great significance. I hope the above can help fellow technicians.

References:

[1] Wang Detian, Concise Pathological Techniques

[2] Ma Henghui, Zhou Xiaojun. Common Problems and Countermeasures in Tissue Sectioning[J]. Journal of Clinical and Experimental Pathology, 2009, 25(002):211-214.

[3] Li Hui, Fu Haiyan, Ma Lili, et al. Coping with Common Problems in Paraffin Tissue Sectioning[J]. Journal of Clinical and Experimental Pathology, 2016, 32(2):232-233.

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