An older carousel tissue processor carries two stacked baskets of cassettes through 12 open reagent beakers. On Sunday a trainee changed every reagent, filling each beaker to 1.6 liters; the manufacturer's fill line for a two-basket load is 2.0 liters. Monday's overnight run held 180 cassettes, filling both baskets. Next morning, blocks from the upper basket have soft, chalky white centers and crumble at microtomy, while blocks from the lower basket cut well. The last absolute ethanol reads 99% on the hydrometer, the wax baths are 2 °C above the wax melting point, and every specimen had at least 24 hours in formalin. The most likely cause is:

Reagent levels too low for the upper basket

Water carried over into the last ethanol

Incomplete formalin fixation of the tissue

Wax baths set too cool for full infiltration

Why are paraffin infiltration baths kept only about 2-4 °C above the wax melting point?

Overheating hardens and shrinks tissue

Overheated wax turns tissue opaque white

Hotter wax draws water back into tissue

Hotter wax leaves xylene trapped in tissue

Blocks from a breast reduction show well-cut fibrous tissue, but the fat is soft, greasy and smears at microtomy. Skin biopsies from the same run cut well. The most likely cause is:

Fat not fully removed during clearing

Over-fixation of the fat in formalin

Sections cut too thin for fatty tissue

Why are small endoscopic biopsies processed on a shorter schedule than large resection blocks?

Long schedules make them hard and brittle

They need no clearing step at all

Short schedules give stronger eosin staining

Long schedules wash out their fixative

Tissue processing: dehydration, clearing and infiltration

8 DOH Abu Dhabi (HAAD) lab licensing practice questions on Tissue processing: dehydration, clearing and infiltration, part of Histopathology and Cytology. The first 8 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
An older carousel tissue processor carries two stacked baskets of cassettes through 12 open reagent beakers. On Sunday a trainee changed every reagent, filling each beaker to 1.6 liters; the manufacturer's fill line for a two-basket load is 2.0 liters. Monday's overnight run held 180 cassettes, filling both baskets. Next morning, blocks from the upper basket have soft, chalky white centers and crumble at microtomy, while blocks from the lower basket cut well. The last absolute ethanol reads 99% on the hydrometer, the wax baths are 2 °C above the wax melting point, and every specimen had at least 24 hours in formalin. The most likely cause is:
1. AWater carried over into the last ethanol
2. BIncomplete formalin fixation of the tissue
3. CWax baths set too cool for full infiltration
4. DReagent levels too low for the upper basket
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Answer: D · Reagent levels too low for the upper basket
Only the upper basket is affected, so the cause must act on part of the load rather than on every cassette in the run. With each beaker 0.4 liters short of the fill line, the top cassettes sat partly or wholly above the reagents, so they were never fully dehydrated, cleared or infiltrated, and the soft, chalky centers are the result. Water in the last ethanol, a cool wax bath or poor fixation would affect both baskets, and the hydrometer, temperature and fixation records are all normal. The soft blocks can be melted down and reprocessed. Reagent volumes should be checked against the fill line for the load before every run, and the trainee shown the correct level.
- A: Water in the last ethanol would affect both baskets, and the hydrometer reads 99%.
- B: Every specimen had at least 24 hours in formalin, and poor fixation would not spare one basket.
- C: A cool wax bath would affect every cassette, and the baths are at the right temperature.
Question 2 · Easy
In routine paraffin processing, the step that directly follows dehydration in graded ethanol is:
1. AClearing in xylene
2. BFixation in formalin
3. CEmbedding in a mold
4. DInfiltration with wax
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Answer: A · Clearing in xylene
The usual order is fixation, dehydration, clearing, then infiltration with molten paraffin, followed by embedding. Clearing replaces the ethanol with a solvent such as xylene that also mixes with molten wax. Each step removes the previous fluid completely before the next one starts. Many processors begin with one or two formalin stations so fixation continues on the machine.
- B: Fixation comes before dehydration.
- C: Embedding is done after processing is finished.
- D: Wax infiltration comes after clearing.
Question 3 · Medium
Why are paraffin infiltration baths kept only about 2-4 °C above the wax melting point?
1. AOverheating hardens and shrinks tissue
2. BOverheated wax turns tissue opaque white
3. CHotter wax draws water back into tissue
4. DHotter wax leaves xylene trapped in tissue
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Answer: A · Overheating hardens and shrinks tissue
Wax held well above its melting point overheats the tissue and makes it hard, brittle and shrunken, so it may crack or crumble at microtomy. Infiltration works well a few degrees above the melting point, for example 58-60 °C for a 56 °C wax. Vacuum and agitation, not extra heat, are used to speed infiltration of dense tissue. Wax baths are checked and recorded daily as part of processor maintenance.
- B: Infiltrated tissue is normally opaque in solid wax; this is not a heat effect.
- C: Water cannot re-enter tissue in a wax bath.
- D: Trapped xylene comes from too few or too short wax changes, not from heat.
Question 4 · Hard
Blocks from a breast reduction show well-cut fibrous tissue, but the fat is soft, greasy and smears at microtomy. Skin biopsies from the same run cut well. The most likely cause is:
1. AFat not fully removed during clearing
2. BOver-fixation of the fat in formalin
3. CSections cut too thin for fatty tissue
4. DA wax bath set too cool
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Answer: A · Fat not fully removed during clearing
Fat must be dissolved out by the dehydrating and clearing agents before wax can replace it, and fatty tissue needs longer times and fresh reagents for this. Where fat remains, wax does not infiltrate, so the tissue is greasy and unsupported and smears or crumbles. Thinner slices, longer or extra clearing changes, and avoiding fat-laden xylene help, and some labs add an isopropanol or defatting step. Skin biopsies in the same run cut well because they contain little fat.
- B: Over-fixation hardens tissue; it does not leave fat greasy.
- C: Section thickness cannot fix tissue that wax has not infiltrated.
- D: A cool wax bath would affect every block in the run, not only the fat.
Question 5 · Medium
Why are small endoscopic biopsies processed on a shorter schedule than large resection blocks?
1. AThey need no clearing step at all
2. BShort schedules give stronger eosin staining
3. CLong schedules wash out their fixative
4. DLong schedules make them hard and brittle
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Answer: D · Long schedules make them hard and brittle
Reagents reach the center of a 1-2 mm biopsy quickly, so an overnight schedule built for 3-4 mm blocks leaves it in alcohols, xylene and hot wax far longer than needed. That over-processing makes small biopsies hard, brittle and difficult to cut thinly. Labs therefore run a separate short schedule for biopsies. Schedule length is set by the thickest, densest or fattiest tissue in the run.
- A: Every paraffin schedule needs clearing, because ethanol and wax do not mix.
- B: Schedule length does not decide eosin intensity.
- C: Bound fixative is not washed out by the processing reagents.
Question 6 · Hard
On an overnight processor, tissue in the first xylene turns milky and the blocks are soft and crumbly at the center. Checks: last 'absolute' ethanol 88% by hydrometer; xylene and wax changed yesterday; wax bath 58 °C for a 56 °C wax. The most likely cause is:
1. AOverheated infiltration wax
2. BFormalin carried into xylene
3. CIncomplete dehydration
4. DToo short a 70% ethanol step
Answer and explanation in LearnFree account
Question 7 · Medium
Dehydration usually begins in 70% ethanol rather than absolute ethanol, mainly to:
1. AComplete the formalin fixation
2. BLet xylene enter the tissue sooner
3. CAvoid the cost of absolute ethanol
4. DLimit shrinkage and distortion of the tissue
Answer and explanation in LearnFree account
Question 8 · Easy
Why is a clearing agent such as xylene used between dehydration and wax infiltration?
1. AXylene completes the fixation
2. BXylene softens hard tissues
3. CEthanol does not remove all the water
4. DParaffin does not mix with ethanol
Answer and explanation in LearnFree account

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Tissue processing: dehydration, clearing and infiltration

In routine paraffin processing, the step that directly follows dehydration in graded ethanol is:

Why are paraffin infiltration baths kept only about 2-4 °C above the wax melting point?

Blocks from a breast reduction show well-cut fibrous tissue, but the fat is soft, greasy and smears at microtomy. Skin biopsies from the same run cut well. The most likely cause is:

Why are small endoscopic biopsies processed on a shorter schedule than large resection blocks?

On an overnight processor, tissue in the first xylene turns milky and the blocks are soft and crumbly at the center. Checks: last 'absolute' ethanol 88% by hydrometer; xylene and wax changed yesterday; wax bath 58 °C for a 56 °C wax. The most likely cause is:

Dehydration usually begins in 70% ethanol rather than absolute ethanol, mainly to:

Why is a clearing agent such as xylene used between dehydration and wax infiltration?