PGT-A vs PGT-SR: What Each Test Looks For and Why the Difference Matters

PGT-A and PGT-SR are both preimplantation genetic tests performed on a small sample of cells taken from an embryo. The similarity ends there. PGT-A asks a broad question about chromosome number across the embryo’s chromosomes. PGT-SR asks a narrow, pre-defined question about chromosomes affected by a structural rearrangement already known to exist in a parent. One is a screening approach; the other is a targeted test built around a specific family finding.

If a clinic has mentioned “PGT” without saying which one, the first useful step is to establish which test is actually being proposed and why. The two are not interchangeable, and PGT-SR is not a more thorough version of PGT-A.

The core difference in one sentence each

• PGT-A looks at chromosome number — aneuploidy — across the embryo’s chromosomes, to identify embryos with an abnormal number of whole chromosomes.
• PGT-SR is designed for embryos from a parent who carries a known chromosomal structural rearrangement, such as a translocation or inversion, and looks for unbalanced chromosome content arising from that rearrangement.

The UK regulator’s patient information describes PGT-A as assessing chromosome number in sampled embryo cells, and separately describes PGT-SR as addressing chromosomal structural rearrangements — different questions requiring different test design. That distinction is the heart of the matter.

Why the indications differ

PGT-A is typically raised in conversations about age-related risk, a history of recurrent pregnancy loss, or certain male-factor situations, where the concern is a random chromosome-number error in the embryo. PGT-SR enters the conversation through a different door: a karyotype result, or another genetic finding, showing that a parent carries a balanced structural rearrangement.

A balanced rearrangement can leave a parent healthy while raising the chance that an embryo inherits an unbalanced amount of chromosome material. PGT-SR is built to look for that specific pattern. It is not a general screen for aneuploidy across all chromosomes, and it does not replace PGT-A unless the treating team has designed it to do both.

Why PGT-SR needs preparation PGT-A does not

PGT-SR cannot be ordered as a generic add-on. It depends on prior information:

1. A documented parental karyotype or equivalent finding identifying the rearrangement and the chromosomes involved.
2. A tailored probe design built around that specific rearrangement, which takes laboratory work before an embryo biopsy can be interpreted.
3. Genetic counselling to explain inheritance patterns, what the test can and cannot detect, and what the results mean for the family.

This is why the practical preparation for PGT-SR often starts months before a cycle, not at the clinic door. If you are travelling internationally, ask early whether your existing karyotype report is sufficient or whether the laboratory needs additional information or a repeat sample before probe design can begin.

What a “normal” result means — and does not mean

	PGT-A	PGT-SR
What is examined	Chromosome number across the embryo’s chromosomes	Chromosome content related to the known parental rearrangement
What “normal” suggests	No aneuploidy detected in the sampled cells	No unbalanced content from the rearrangement detected in the sampled cells
What it cannot confirm	That the whole embryo is normal, or that pregnancy will follow	That the embryo is free of all chromosome issues, or that pregnancy will follow

Both tests work from a biopsy sample, not the whole embryo. The regulator notes that PGT-A has limitations and can produce incorrect findings, and that a result does not guarantee a successful pregnancy or a healthy child. The same caution applies to PGT-SR: a reassuring result reduces one specific concern but does not certify the embryo.

Mosaic findings add another layer. A mosaic PGT-A result is inferred from intermediate chromosome copy-number findings in a biopsy sample, and technical factors can contribute to such findings. A sampled result does not establish the cell composition of the whole embryo. Results and their uncertainty should be discussed with the treating clinician and a genetic counsellor.

Practical preparation for international patients

Before comparing quotes or booking travel, clarify these points in writing:

• Which test is being quoted. Ask for the test name, the chromosomes it covers, and whether PGT-A and PGT-SR are being combined or offered separately.
• Whether a parental karyotype is needed first. For PGT-SR, confirm whether your existing report is accepted and whether probe design must be completed before a cycle starts.
• What records to gather. Karyotype reports, genetic counselling notes, previous PGT reports and any relevant family history should travel with you in dated copies.
• Who explains the results. Confirm whether results are discussed by the treating clinician, a genetic counsellor, or both, and in what language.
• What happens if the test is inconclusive. Ask how a failed or uninformative result would be handled and whether additional costs apply.

Patients can and should ask licensed clinicians about diagnosis, test options, risks, reports and individual chances. A coordination service can help organise records, request appointments and compare what each quote includes, but it does not diagnose, interpret results or recommend a particular test.

Where the two tests sit alongside PGT-M

PGT-M addresses a known monogenic condition, PGT-SR addresses a known structural rearrangement, and PGT-A addresses chromosome number. Each answers a different question, and none is a universal requirement. Whether any of them is appropriate depends on your history, your records and a clinical assessment — not on a general claim that more testing is better.

If you are weighing a previous IVF plan against a new proposal, it helps to compare the tests named in each plan side by side, including what each was meant to detect. A structured comparison of recommendations can make those differences visible before a consultation.

Frequently asked questions

Can PGT-SR also detect aneuploidy?

PGT-SR is designed around a specific parental structural rearrangement and the chromosomes involved. Whether a clinic's PGT-SR protocol also reports general chromosome-number findings varies by laboratory and test design, so ask the treating team exactly what a given report will and will not cover rather than assuming.

Do I need a karyotype before starting PGT-SR?

PGT-SR depends on a documented parental rearrangement so that a tailored probe can be designed. Ask the laboratory whether your existing karyotype report is sufficient or whether additional information or a repeat sample is needed, and how long probe design takes before a cycle can begin.