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Homologous Recombination Deficiency (HRD) Test: DNA Repair Deficiency, Ovarian Cancer, and Genomic Score

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Learn what an HRD test measures in ovarian cancer, how genomic instability scores and BRCA status are interpreted, what HRD-positive means, and how results can inform PARP inhibitor treatment decisions.

A homologous recombination deficiency, or HRD, test looks for evidence that a tumor has lost an important high-accuracy DNA repair pathway. In ovarian cancer, HRD testing is used mainly to help estimate how likely a tumor is to benefit from certain maintenance strategies, especially treatments involving PARP inhibitors. The test may combine BRCA1/BRCA2 mutation analysis with a genomic instability or “genomic scar” score that reflects patterns of past DNA repair failure.

HRD is not one gene and there is no single universal HRD score. Different commercial and laboratory-developed assays use different genes, mathematical methods, and cutoffs. A result may be reported as HRD-positive, HRD-negative, or inconclusive, sometimes with a numeric genomic instability score. HRD-positive does not automatically mean an inherited BRCA mutation, and HRD-negative does not mean a tumor can never respond to platinum or a PARP inhibitor. The result is most useful when interpreted with tumor type, BRCA status, treatment history, specimen quality, and the specific assay used.

  • HRD means the tumor shows evidence of impaired homologous recombination DNA repair, often from BRCA1/2 dysfunction or related mechanisms.
  • An HRD-positive ovarian cancer is generally more likely to benefit from PARP-inhibitor-based maintenance than an HRD-negative tumor, but the result is not a perfect predictor.
  • A genomic instability score is assay-specific; there is no single normal or positive cutoff that applies to every HRD test.
  • HRD-positive does not necessarily mean hereditary cancer because the abnormality may be confined to the tumor.
  • Low tumor content, damaged tissue, prior treatment, or assay failure can produce an inconclusive result that is not equivalent to HRD-negative.

Table of Contents

What homologous recombination deficiency means

Cells constantly develop DNA damage. One of the most serious forms is a double-strand break, in which both strands of the DNA molecule are damaged. Homologous recombination repair is a high-fidelity pathway that uses a matching DNA template to repair this damage accurately.

A tumor is described as homologous recombination deficient when this pathway no longer works effectively. The defect creates genomic instability and forces cancer cells to depend more heavily on alternative repair mechanisms.

Several biological events can produce HRD. They include:

  • Pathogenic BRCA1 or BRCA2 mutations.
  • Loss or inactivation of the remaining normal BRCA copy in tumor cells.
  • BRCA1 promoter methylation, which can silence the gene without changing its DNA sequence.
  • Alterations in other homologous recombination repair genes.
  • More complex or incompletely understood mechanisms that leave a characteristic pattern of genomic damage.

The key concept is that HRD is a tumor phenotype, not simply the name of one mutation. Two tumors can both be HRD-positive for different biological reasons. One may have a BRCA1 mutation; another may have no BRCA mutation but still show extensive genomic scarring consistent with homologous recombination failure.

This biology creates a therapeutic vulnerability. PARP inhibitors interfere with another DNA repair process. Tumor cells already impaired in homologous recombination can be especially sensitive to this additional repair stress, a principle often described as synthetic lethality.

The same DNA repair context can also contribute to sensitivity to platinum chemotherapy. However, neither platinum response nor HRD status is absolute. Tumors can develop resistance, and biomarkers do not perfectly predict what will happen in an individual patient.

What an HRD test measures

HRD assays generally use one or both of two approaches: cause-based testing and genomic scar testing.

BRCA and homologous recombination repair gene testing

Cause-based testing looks for pathogenic alterations in genes involved in homologous recombination, especially BRCA1 and BRCA2. Some assays also evaluate additional DNA repair genes. Finding a tumor BRCA1/2 pathogenic mutation can establish an important HRD mechanism, but testing a long list of repair genes does not automatically identify every HRD tumor because the functional significance of different genes varies.

Tumor BRCA analysis should not be confused with dedicated hereditary testing. A BRCA1 mutation test or BRCA2 mutation test performed on blood or saliva can evaluate germline risk, while tumor sequencing can detect both inherited and tumor-only alterations.

Genomic scar or genomic instability testing

When homologous recombination fails over time, the tumor genome develops characteristic patterns. HRD assays can quantify these patterns. Common components include:

  • Loss of heterozygosity (LOH): large regions lose one parental DNA copy.
  • Telomeric allelic imbalance (TAI): unequal chromosome segments extend toward chromosome ends.
  • Large-scale state transitions (LST): numerous large chromosomal breaks or transitions appear across the genome.

Some established assays combine these features into a genomic instability score. A commonly encountered ovarian-cancer cutoff is 42 on certain validated assay versions, but this number must not be treated as a universal rule. Other platforms use different algorithms, thresholds, or definitions of HRD positivity.

That is why the report should always name the assay. A score of 40 has no reliable meaning without knowing whether the laboratory’s validated cutoff is 42, another value, or a completely different metric.

How to read HRD-positive, HRD-negative, and genomic score results

An HRD report usually provides an overall classification plus supporting details such as BRCA status and a genomic score.

HRD-positive

An HRD-positive result means the tumor meets the assay’s validated definition of homologous recombination deficiency. Depending on the platform, positivity may come from a pathogenic BRCA1/2 alteration, a genomic instability score above the assay cutoff, or either criterion.

In ovarian cancer, HRD-positive status generally identifies a group with greater average benefit from PARP-inhibitor-based maintenance strategies than HRD-negative disease. It does not guarantee response, and it does not by itself determine which drug is appropriate.

HRD-negative

An HRD-negative result means the tumor did not meet that assay’s threshold for deficiency. It does not prove that homologous recombination is fully normal in every cancer cell, nor does it mean treatment can never work.

Clinical trials have shown that the size of PARP inhibitor benefit often differs by biomarker subgroup. Treatment decisions therefore use the HRD result together with BRCA status, stage, response to platinum, prior treatment, toxicity considerations, and the regulatory or guideline context for a specific regimen.

Inconclusive, failed, or insufficient

An inconclusive result is not the same as HRD-negative. It can occur because too little tumor is present, DNA is degraded, the specimen fails quality thresholds, or the algorithm cannot generate a reliable score.

If HRD status would change management, the team may consider another tumor block or a different specimen. Whether retesting is worthwhile depends on tissue availability and timing.

Interpreting the numeric score

A genomic score is best viewed as an assay output used to create a category, not as a continuous wellness number. A score just below a cutoff does not mean a tumor has “almost normal” DNA repair, and a very high score does not prove a larger treatment benefit for an individual patient.

The validated positive/negative interpretation, the assay name, and the clinical setting are more important than comparing raw scores between different platforms.

HRD testing in ovarian cancer

HRD testing is most established in high-grade epithelial ovarian cancer, particularly high-grade serous and related advanced cancers where maintenance treatment decisions follow response to platinum-based therapy.

The practical goal is to identify tumors with DNA-repair biology associated with greater sensitivity to PARP inhibition. Evidence from major trials has shown substantial benefit from PARP-inhibitor-based maintenance in HRD-positive groups. In the PAOLA-1 study, for example, first-line maintenance olaparib plus bevacizumab showed a clinically meaningful long-term survival benefit in the HRD-positive subgroup compared with bevacizumab alone.

This does not mean every HRD-positive patient should receive the same maintenance treatment. The appropriate option depends on factors such as:

  • Whether the tumor has a BRCA mutation.
  • Whether bevacizumab was part of first-line treatment.
  • Response to platinum chemotherapy.
  • Prior PARP inhibitor exposure.
  • Disease setting and line of therapy.
  • Contraindications, toxicity risks, and patient preferences.
  • Current drug approvals and professional guidance.

HRD can therefore be thought of as a predictive enrichment biomarker: it helps identify a population in which benefit is more likely or greater on average. It is not a stand-alone prescription.

Testing is often ordered early enough for results to be available when first-line maintenance is discussed. The pathology team may select a diagnostic biopsy or surgical specimen with adequate tumor. Because HRD assays require more DNA and more genome-wide information than a simple single-gene test, specimen planning matters.

When HRD testing is most useful

The highest-value time for HRD testing is usually when the result can change a concrete ovarian-cancer decision. In newly diagnosed advanced disease, that often means obtaining the result before maintenance therapy is selected after platinum-based treatment. Waiting until the maintenance decision is due can create avoidable delays if tissue retrieval, pathology review, or repeat testing is required.

HRD testing is not a screening test for people without cancer and is not used to diagnose an ovarian mass by itself. It is a biomarker test on an established tumor. It also should not be ordered simply to generate a score when the result would not affect management. The clinical team should know which treatment question the assay is intended to answer.

In recurrent disease, the value of a new HRD assay is more nuanced. The original genomic scar may still be detectable even after treatment has changed the tumor’s current repair function. Prior platinum response, prior PARP inhibitor exposure, resistance mechanisms, and an updated tumor profile may provide information that a static scar score cannot capture alone.

The HRD result is also interpreted alongside germline and somatic BRCA testing. In ovarian cancer, hereditary evaluation has implications that extend beyond treatment, including future cancer risk and relatives’ health.

If a report lists both “tumor BRCA” and “HRD status,” read both lines. A BRCA-mutated tumor is an important HRD subgroup, while a BRCA-wild-type tumor can still be HRD-positive through genomic instability. That distinction may matter when the oncology team compares evidence from trials or drug labels that define biomarker groups differently.

Specimen requirements and test limitations

Most clinical HRD assays use formalin-fixed, paraffin-embedded tumor tissue. A pathologist selects a block with enough viable tumor and may mark an area for enrichment. The laboratory then extracts DNA and evaluates sequencing and genomic instability metrics.

Tissue quality is a major practical issue. Factors that can reduce test success include:

  • Low tumor cellularity.
  • Small biopsies.
  • Extensive necrosis.
  • Poor fixation or degraded DNA.
  • Specimens with large amounts of normal tissue.
  • Treatment-related changes that leave little viable tumor.

Real-world studies show that optimal specimen selection can materially reduce HRD test failure. When several blocks are available, the most recent specimen is not automatically the best; a high-quality pretreatment specimen with abundant tumor may perform better than a poorly cellular post-treatment sample.

HRD testing also has biological limitations. Genomic scars are historical. They show that the tumor accumulated damage consistent with homologous recombination failure, but they may not perfectly measure current repair function. A tumor can restore homologous recombination through a resistance mechanism, such as a BRCA reversion, while the old genomic scar remains visible.

This issue matters especially in recurrent disease after multiple therapies. A scar-based test can remain HRD-positive even when the tumor has evolved and become less sensitive to PARP inhibition or platinum.

The opposite problem can also occur: an assay may classify a tumor as HRD-negative even though an unmeasured mechanism impairs repair. No current HRD test captures every possible biological route to deficiency.

Platform differences create another limitation. Assays can differ in gene content, genomic regions measured, scoring algorithms, cutoff values, and validation populations. Results from two different assays are not necessarily interchangeable.

HRD vs BRCA and hereditary risk

HRD-positive is not synonymous with germline BRCA-positive. This is one of the most important interpretation points for patients and families.

A tumor can be HRD-positive because of:

  • A germline BRCA1 or BRCA2 pathogenic variant.
  • A somatic BRCA1/2 mutation found only in the tumor.
  • BRCA1 promoter methylation.
  • Another repair-pathway alteration.
  • Genomic instability without an identified BRCA cause.

Therefore, an HRD report cannot by itself answer whether a cancer predisposition was inherited. Germline testing uses a blood, saliva, or other non-tumor sample and is interpreted under hereditary cancer guidelines.

Conversely, a person with a germline BRCA pathogenic variant can have important implications beyond the current ovarian tumor. The result may affect future breast or other cancer risk, preventive strategies, and whether adult relatives should consider targeted genetic testing.

For ovarian cancer, professional recommendations support appropriate germline and somatic testing rather than using HRD as a substitute. The tests answer different questions: HRD asks about tumor repair biology; germline testing asks about inherited risk.

A variant of uncertain significance in BRCA1, BRCA2, or another repair gene also should not be treated as though it proves hereditary risk or HRD. Laboratories classify variants according to available evidence, and uncertain findings may be reclassified over time.

Questions to ask about an HRD result

The most useful questions focus on the assay, the specimen, and what the result changes clinically.

  1. Which HRD test was used? The platform determines what genes, genomic scars, and cutoff were evaluated.
  2. Is the tumor HRD-positive because of BRCA, the genomic score, or both? This helps separate the underlying evidence.
  3. What is the assay-specific cutoff? Do not compare the numeric score with a threshold from another platform.
  4. Was the specimen adequate and did all quality checks pass? A failed or inconclusive result should not be called negative.
  5. Does this result change first-line maintenance treatment? The answer depends on the exact ovarian cancer setting and prior therapy.
  6. Has germline BRCA testing been performed? Tumor HRD testing cannot determine inherited risk by itself.
  7. Could prior treatment make a scar-based result less predictive now? This may matter in recurrent or PARP-resistant disease.
  8. Would another specimen be useful if the result is inconclusive? A better block can sometimes rescue testing.

HRD testing is most helpful when it is treated as one component of precision oncology rather than a binary verdict. It captures important DNA repair biology, but the strongest interpretation combines the genomic result with BRCA status, the ovarian cancer subtype, treatment history, and the current evidence for the exact maintenance strategy being considered.

When comparing HRD results over time, keep the assay name and cutoff with the result. A genomic instability score from one platform should not be converted into the threshold used by another platform, even when both reports use the phrase “HRD positive.” The assays may examine different genomic scar features, gene sets, and quality-control rules. This matters when an older report is reviewed at a new cancer center: the useful question is whether that exact validated assay result applies to the treatment decision being considered, not whether its raw number looks high or low on a different scale.

References

Disclaimer

This article is for general educational purposes and does not replace advice from a gynecologic oncologist, pathologist, or genetics professional. HRD assays differ in methods and cutoffs, and treatment relevance depends on the ovarian cancer subtype, BRCA status, prior therapy, specimen quality, and current regulatory or guideline criteria. HRD testing does not replace dedicated germline testing for inherited cancer risk.

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