SCSA vs TUNEL vs SCD: Reading a Sperm DNA Test Report

By Dr. Lejla Fazlicic, D.Ac., L.Ac. · Licensed Acupuncturist, Illinois · About the author

Evidence-based · Written by Dr. Lejla Fazlicic, D.Ac, L.Ac · All key claims cited to peer-reviewed research · Last reviewed October 2026

The short answer: SCSA, TUNEL, SCD (often sold as Halosperm) and the Comet assay are four ways of measuring the same thing: the share of sperm with broken DNA. They use different lab methods, and their numbers aren't interchangeable, so each test has its own cutoffs [1]. The test name on the report tells you which scale your number sits on. When you retest, use the same test, ideally at the same lab.

A sperm DNA report can look like it was written for the lab, not for you: an acronym, a percentage, sometimes a second number with no explanation. This article walks through what each test name means, what each line on a report usually is, and what to ask the clinician who ordered it. It doesn't interpret anyone's result. That's the ordering clinician's job.

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What do SCSA, TUNEL, SCD and Comet measure?

All four estimate how many sperm in the sample have breaks in their DNA. None of them tells single-strand breaks from double-strand breaks [1]. The difference is how the lab finds the breaks.

The four main sperm DNA tests (Esteves and Humaidan, 2026)
Test name on the report	How it works, in plain words	Worth knowing
SCSA (sperm chromatin structure assay)	The DNA is exposed to mild acid and stained, then a machine (a flow cytometer) reads thousands of sperm one at a time.	Highly reproducible, works on fresh or frozen samples, needs specialist equipment.
TUNEL	An enzyme attaches a glowing tag directly to the broken ends of DNA.	Quantitative, labor-intensive.
SCD (sperm chromatin dispersion, often sold as Halosperm)	Sperm are treated so that intact DNA spreads out into a "halo" under the microscope. Sperm with damaged DNA show a small halo or none.	Simple and lower-cost. Manual scoring depends on the person counting; automated reading helps.
Comet (alkaline)	An electric current pulls broken DNA pieces out of each sperm, which makes a "comet tail".	Varies more between labs.

Some at-home mail-in kits use SCD. Legacy, for example, says its kit uses the Halosperm SCD method [4]. A mail-in result belongs on the same scale as any other SCD result, not the SCSA or TUNEL scale.

Why can't you compare a TUNEL number with an SCSA number?

The tests put men in similar groups (lower, middle, higher) fairly well, but their actual percentages don't line up. A 2026 clinical review says that "absolute values are not interchangeable" between tests [1]. That's why each test has its own published cutoffs, for example about 30–35% for SCSA and 36% for TUNEL in the context of IVF and ICSI. Our article on what a normal DFI is lists them all.

In practice, a result of 28% on one test and 24% on another, months apart, doesn't tell you whether anything changed. Only a retest on the same test can do that.

What does each line on a sperm DNA report mean?

Test name. Usually near the top: SCSA, TUNEL, SCD or Halosperm, or Comet. This tells you which set of cutoffs applies.
DFI (DNA fragmentation index). The percentage of sperm counted that had DNA breaks. Some reports call it SDF.
HDS (high DNA stainability). Only on SCSA reports. It measures sperm whose DNA wasn't packed tightly, which is often a sign of immature sperm. It's separate from DFI [2].
Abstinence days. How many days since his last ejaculation. A 2026 review recommends testing after 2–3 days, because longer gaps are linked to higher fragmentation [1]. If it says 7 days, mention that to the clinician.
The lab's reference range. The lab's own cutoffs. These come before any number you read online.

What the guidelines say

A 2026 review recommends that each lab use one validated test and set its own reference ranges. Reports should state the test used and the abstinence period [1].
The same review calls the published cutoffs "decision-support tools, not absolute cut-offs", to be read together with the female partner's picture [1].
The American Urological Association and ASRM advise against DNA fragmentation testing in a couple's first fertility workup. They do recommend it for the male partner when a couple has had recurrent pregnancy loss [3].

Is one sperm DNA test better than the others?

No single test is the clear winner. Each has strengths, and the review treats all four as valid ways to measure overall DNA breaks [1]. SCSA is highly reproducible: its developer reports that the same samples give near-identical results on different labs' machines [2]. SCD is simpler and cheaper. For most couples, the more useful question isn't "which test is best?" but "which test does our clinic use and know how to read?"

What to ask the clinician who ordered it

Which test was this, and what cutoff does this lab use?
Was the abstinence period in the recommended range?
Does this change anything in our plan?
Should he see a urologist about possible causes?
If we retest, when, and with the same test and lab?

A 2026 review suggests retesting about 10–12 weeks after the first test when checking for change, which is about one full cycle of sperm production [1]. Our sperm DNA test cost guide covers how to get tested and what it usually costs.

What the research can't tell you

What your number means for your cycle. Cutoffs are statistical. The SCSA developer reports full-term natural pregnancies with DFI as high as 68% [2].
Whether two results from different tests show progress. They're on different scales.
Who is to blame. A result is information, not a verdict on anyone.

What we do with this

Find the test name and the abstinence days on the report before reading the number.
Take the five questions above to the clinician who ordered it.
Use the time before any retest well. The daily habits linked to sperm DNA health are covered in the 74-day habits guide. His Pocket Coach turns them into small daily steps.

In my clinical experience, men feel very differently about a report once they can read it. It stops being a judgment and becomes a starting point. Try his Pocket Coach free for 7 days.

Frequently asked questions

What is an SCSA test?

SCSA stands for sperm chromatin structure assay. The lab exposes sperm DNA to mild acid, stains it, and uses a flow cytometer to read thousands of sperm one at a time. It reports DFI, the percentage of sperm with DNA breaks, and HDS, a measure of sperm whose DNA wasn't packed tightly.

What is a TUNEL assay for sperm DNA fragmentation?

TUNEL uses an enzyme to attach a glowing tag directly to the broken ends of sperm DNA, then counts how many sperm are tagged. Its cutoffs differ from SCSA's: a 2026 review lists about 36% for TUNEL in the context of IVF and ICSI.

Is Halosperm the same as SCD?

Yes. Halosperm is a commercial kit for the SCD (sperm chromatin dispersion) test. Sperm with intact DNA form a halo under the microscope, and sperm with damaged DNA show a small halo or none. Some mail-in kits, including Legacy's, use this method.

Can I compare results from two different sperm DNA tests?

Not directly. The tests group men similarly, but their percentages aren't interchangeable, and each has its own cutoffs. To see whether anything changed, retest with the same test, ideally at the same lab, about 10–12 weeks later.

What is HDS on a sperm DNA report?

HDS stands for high DNA stainability and appears only on SCSA reports. It measures sperm whose DNA wasn't packed tightly, often a sign of immature sperm, and it's separate from DFI. Ask the clinician who ordered the test what it means for your plan.

References

Esteves SC, Humaidan P. Sperm DNA fragmentation: how to test, when to test, and what to do with abnormal results — a pragmatic mini-review for clinical practice. Human Reproduction. 2026;41(7):1024–1039. https://academic.oup.com/humrep/article/41/7/1024/8664338
Evenson DP. Evaluation of sperm chromatin structure and DNA strand breaks is an important part of clinical male fertility assessment. Translational Andrology and Urology. 2017. https://pmc.ncbi.nlm.nih.gov/articles/PMC5643675/
American Urological Association / American Society for Reproductive Medicine. Diagnosis and Treatment of Infertility in Men: AUA/ASRM Guideline (2020; amended 2024). https://www.auanet.org/guidelines-and-quality/guidelines/male-infertility
Legacy. Frequently asked questions: sperm DNA fragmentation analysis method. https://www.givelegacy.com/faq

This article is educational and reflects associations reported in the scientific literature. It is not medical advice, not a diagnosis, and not a way to interpret your own results. As a Doctor of Acupuncture I offer lifestyle guidance that works alongside your physician and fertility clinic — I do not diagnose, interpret labs, or prescribe. Your clinic may have its own protocol; follow it and raise any questions with your medical team. Cycle Decoded has no affiliation with any testing company mentioned.

About the author

Dr. Lejla Fazlicic, D.Ac, L.Ac is a Doctor of Acupuncture and Illinois-licensed acupuncturist with 15 years of fertility-focused practice, including two years at Pulling Down the Moon in Chicago. She works with both partners in the months before IVF on the daily-life factors that shape sperm development and final egg maturation. Work with Dr. Lejla

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