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Blocks and slides

Paraffin Blocks and Slides: — The Physical Record of Your Diagnosis

When a biopsy is taken, the tissue is not stored as a photograph or a document. It is preserved in a small wax cube called a paraffin block, and thin slices from that block are placed on glass slides. Those two objects are where your diagnosis actually lives.

Medically reviewed by Dr. T. Raghavender Reddy, Medical Oncologist, MBBS · DM (Medical Oncology) · MD (Radiation Oncology) · Last reviewed September 2026

  • Not just a report — The block and slides are the source material. The report is the pathologist's interpretation of them.
  • Can be retested — New slices can be cut from the block if additional tests are needed later.
  • Can travel — The slides and block can be sent to another pathologist for a second opinion review.
  • Kept for years — Laboratories are required to store blocks and slides for a defined period after your biopsy.
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A paraffin block is your biopsy tissue embedded in a small wax cube. Glass slides are thin slices cut from that block, stained so cell structures are visible under a microscope. The pathologist reads the slides to reach a diagnosis. The block is the archive — it can be re-cut for further testing or sent for a second opinion.

What is the difference between a block and a slide?

Paraffin blockGlass slide
What it isBiopsy tissue set inside a small wax cubeA thin slice cut from the block, stained and mounted on glass
AppearanceSmall, firm cube in a labelled plastic cassetteA glass rectangle about the size of a credit card, with tissue visible on it
How the tissue is heldSealed inside wax — durable and stable over timeExposed on glass — more fragile than the block
What it is used forSource for further cuts, new tests, and second opinionsRead under a microscope by the pathologist to reach the diagnosis
Which comes firstThe block is made firstSlides are cut from the block

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Why do the block and slides matter beyond the first diagnosis?

The report your oncologist received is the pathologist's interpretation of what they saw on the slides. The slides themselves are the evidence behind that interpretation.

If your oncologist wants a second opinion, or if a new test becomes relevant to your treatment, the laboratory needs the physical block to cut fresh sections. A copy of the report is not enough.

This is also why the block matters over time. A test that did not exist when you were first diagnosed may become important later. The block makes that possible.

How does a biopsy become a block and then slides?

  1. Biopsy is taken

    A doctor removes a small piece of tissue from the suspicious area — with a needle, during a procedure, or in surgery.

  2. Tissue is fixed

    The sample is placed in formalin, a chemical that stops cell decay. This preserves the tissue structure exactly as it was at the moment of biopsy.

  3. Water is replaced with wax

    The laboratory processes the tissue through a series of chemicals that remove water and replace it with liquid paraffin wax.

  4. The block is formed

    The wax hardens around the tissue in a small plastic mould, creating the paraffin block. It is labelled and stored at room temperature.

  5. Slides are cut

    A specialist cuts very thin sections from the block using an instrument called a microtome and floats each section onto a glass slide.

  6. Slides are stained

    Dyes are applied so that cell structures take on colour and become visible under a microscope.

  7. Pathologist reads the slides

    The pathologist examines the slides and writes their findings in the report your oncologist receives.

Explore 112 more Markers, Molecular Testing and Test Accuracy topics

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All Markers, Molecular Testing and Test Accuracy →

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Common questions

Frequently asked questions

Can I ask for my block and slides?

Yes. You or your treating doctor can formally request the block and slides from the laboratory that processed your biopsy. This is most often done when seeking a second opinion at another hospital or when additional testing is needed. The laboratory may require a written request and some charge a handling fee. They will usually lend rather than permanently release the block, and expect it to be returned after review.

What if the laboratory cannot find my block?

This is rare but does happen, particularly for older biopsies or when a laboratory has relocated or closed. If the block cannot be found, a second opinion becomes more difficult because the reviewing pathologist has only the existing slides to work from — and slides may not always support the full range of additional tests. If you are planning a second opinion, requesting your block sooner rather than later reduces this risk.

How long are blocks and slides kept?

Retention periods are set by regulatory and accreditation requirements and vary by laboratory type and diagnosis. For most cancer biopsies, blocks are kept for many years. Slides are generally kept for a similar period but may degrade over very long storage. If you are concerned about your specific sample, ask the laboratory directly — they can confirm the expected retention period and the current status of your material.

Why would the treatment team need a new cut from the block?

New cuts are needed when additional tests were not done at the time of the original diagnosis. This is common when a new treatment option becomes available that requires a specific biomarker test, when a molecular panel is ordered, or when a second-opinion pathologist needs more material than the original slides provide. Each cut uses a small amount of the block, so the number of cuts that can be made is finite — though for most biopsies there is enough material for several rounds of testing.

Is the pathology report the same as the slides?

No. The report is the pathologist's written interpretation — their conclusions about what the tissue showed. The slides are the physical evidence those conclusions were drawn from. When a second opinion is sought, the reviewing pathologist looks at the slides themselves, not just the written report. They may reach the same conclusion, add detail, or reach a different interpretation. Both the report and the slides are part of your medical record.

Full index

Browse all 701 biopsy topics

Every page in this section, grouped by the part of the journey it belongs to. Pick a group to see what is in it.

What Is a Biopsy?

What Is a Biopsy? Everything You Need to Know →

Types of Biopsy Compared

Which Biopsy Will You Have? Techniques Compared →

Preparing for a Biopsy

What to Expect on the Day of Your Biopsy →

Recovery and Aftercare

After a Biopsy: Recovery, Aftercare and Warning Signs →

Biopsy by Body Part

Biopsy by Body Part: What to Expect at Each Site →

Understanding Your Report

How to Read a Biopsy Report: Terms Explained →

IHC and Molecular Markers

IHC and Molecular Markers on a Biopsy: What They Mean →

Grading and Scoring Systems

Cytology & Prostate Scoring Systems Explained →

How Accurate Is a Biopsy?

How Accurate Is a Biopsy? Errors and Second Opinions →

If Your Result Is Benign

Your Biopsy Is Benign: What It Means and What Comes Next →

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