Can PGT Screen for Chromosomal Abnormalities in China? Detection Boundaries of PGT-A and PGT-SR

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1. Can PGT Screen for Chromosomal Abnormalities: Answers and Boundaries

In reproductive medicine clinics, when patients consult with a history of recurrent miscarriage or known chromosomal abnormality reports, the core of each decision is whether to recommend PGT and what problems PGT can solve. Answering this question requires first understanding the technical types of PGT and the clinical application standards in China.

Direct answer: Yes, but it depends on the type of PGT and the type of chromosomal abnormality. PGT-A (aneuploidy screening) can detect embryonic chromosomal numerical abnormalities; PGT-SR (structural rearrangement screening) can detect chromosomal structural abnormalities. However, PGT is not a whole-genome screening; for microdeletions/duplications, single-gene disorders, etc., PGT-M or other specialized techniques are required.

1.1 Why This Question Arises

Many patients, and even some clinicians, overestimate the detection range of PGT. On one hand, PGT is often broadly referred to as "third-generation IVF," creating the misconception that it can "test for everything"; on the other hand, reproductive centers may emphasize the advantages of PGT in their education materials without sufficiently addressing its technical boundaries. In reality, PGT's detection capability is determined by the number of biopsied cells, amplification technology, sequencing platform, and data analysis algorithms, and has clear physical and biological limits.

1.2 Physician's Perspective: The Role of PGT in Chromosomal Screening

From a clinical decision-making perspective, PGT is an embryo screening tool, not a diagnostic tool. Its value lies in prioritizing the exclusion of chromosomal numerical abnormalities and large fragment structural abnormalities, thereby reducing the risk of miscarriage and birth defects after transfer. However, a normal PGT result does not guarantee the embryo is healthy; an abnormal PGT result does not mean the embryo cannot develop into a healthy baby—phenomena such as mosaicism and self-correction do exist during embryonic development. Therefore, PGT results must be interpreted in conjunction with embryo morphological grading, patient age, and previous pregnancy history.

2. Comparison of Detection Ranges of Three PGT Technology Types

Clinical Tip: PGT-A and PGT-SR can directly screen for chromosomal abnormalities, but each has technical boundaries. PGT-M does not screen for chromosomal abnormalities but can be used in combination with PGT-A.

3. Indications and Limitations of PGT Clinical Application in China

In China, the implementation of PGT technology strictly follows the "Technical Specifications for Preimplantation Genetic Testing (2021 Revised Edition)". Only tertiary-level A medical institutions approved by the National Health Commission or provincial-level key reproductive medicine laboratories can perform PGT. Clinical indications include:

• Chromosomal numerical abnormalities: Such as aneuploidies like Down syndrome, Edwards syndrome, Patau syndrome.
• Chromosomal structural abnormalities: Such as balanced translocations, Robertsonian translocations, inversions, etc., deemed suitable for PGT-SR after genetic counseling evaluation.
• Recurrent spontaneous miscarriage: Two or more early miscarriages, with normal karyotype analysis of both partners or carrying cryptic translocations.
• Advanced maternal age (≥38 years): Significantly increased oocyte aneuploidy rate; PGT-A can reduce miscarriage rate.
• Severe male factor infertility: Such as non-obstructive azoospermia, potentially related to chromosomal microdeletions.

Not indicated for: General infertile population without clear indications, solely for sex selection, or those with severe mental illness or inability to cooperate with genetic counseling.

3.1 Differences in Detection Capabilities Among Medical Institutions

There are differences in detection platforms and data analysis capabilities among domestic PGT centers. Some centers use high-throughput sequencing (NGS) platforms capable of detecting microdeletions <10 Mb; others primarily use chromosomal microarray (CMA) with lower resolution. Additionally, thresholds for determining mosaicism and reporting standards are not yet fully unified across laboratories, which can affect the positive predictive value of PGT-A. Patients should understand the technology platform and quality control data of their chosen center before making decisions.

4. PGT Testing Process and Timeline

The complete PGT process, from ovulation induction to obtaining results, typically takes 4-6 weeks and involves the following key steps:

1. Ovulation induction and egg retrieval: Approximately 10-14 days to obtain oocytes.
2. In vitro fertilization and blastocyst culture: Blastocysts form on days 5-7 after egg retrieval.
3. Embryo biopsy: Aspiration of 5-10 cells from the trophectoderm layer of the blastocyst.
4. Whole genome amplification and testing: Biopsied cells are amplified, followed by NGS or microarray analysis.
5. Genetic counseling and report interpretation: Test reports are typically issued 10-14 days after biopsy.
6. Frozen embryo transfer: Embryos with normal test results are transferred in a subsequent cycle.

Throughout the cycle, patients need to cooperate in completing genetic counseling, signing informed consent forms, and submitting karyotype analysis reports of both partners. Some centers also require family validation (needed for PGT-SR/PGT-M).

5. Most Easily Overlooked Details

In the clinical application of PGT, several details are easily overlooked but directly impact the accuracy of test results and clinical decision-making:

• Mosaicism issue: Approximately 2%-5% of embryos have chromosomal mosaicism, meaning both normal and abnormal cells exist in the same embryo. PGT-A may report mosaicism as "low-level aneuploidy" or "normal," leading to misjudgment.
• Biopsy timing: Biopsy too early (before day 5) may increase the risk of embryo damage; biopsy too late (after day 7) may affect blastocyst survival rate.
• Amplification failure: About 1%-3% of biopsy samples fail to yield results due to DNA amplification failure, requiring re-biopsy or discarding the embryo.
• Maternal contamination: If maternal cells are mixed in during biopsy, it can distort test results, especially causing false negatives in PGT-A.
• Test report validity: PGT test results are only valid for that specific batch of embryos and cannot be generalized to other cycles.

6. Frequently Asked Questions

7. Practitioner Observation: Common Cognitive Misconceptions in PGT Application

In years of clinical work, I have observed the following typical misconceptions that deserve attention from both patients and clinicians:

Misconception 1: PGT-A can replace prenatal diagnosis

This is the most dangerous misconception. PGT-A is embryo screening, not prenatal diagnosis. After transferring a PGT-A normal embryo, prenatal screening (NIPT) or prenatal diagnosis (amniocentesis) is still required because PGT-A cannot detect all chromosomal abnormalities and has technical errors.

Misconception 2: PGT-SR can repair chromosomal structural abnormalities

PGT-SR only selects embryos carrying normal or balanced chromosomes; it cannot repair abnormalities. Carriers of chromosomal structural abnormalities are themselves healthy but may produce unbalanced gametes when having offspring. PGT-SR helps select normal embryos but does not change the carrier's chromosomal status.

Misconception 3: Earlier PGT is always better

PGT requires clear medical indications for the patient. For young patients without indications, PGT-A not only fails to improve live birth rates but may also reduce the number of usable embryos due to biopsy damage or testing errors. The decision for PGT should be based on individualized assessment, not on the psychology of "feeling better having done it."

8. Differences in PGT Decision-Making by Age Group

Age is a core factor influencing the degree of benefit from PGT-A:

• <35 years: Aneuploidy rate is relatively low (about 20%-25%); the benefit of PGT-A is limited unless there are indications of recurrent miscarriage or genetic disease.
• 35-37 years: Aneuploidy rate rises to 30%-35%; PGT-A can reduce miscarriage rate, but the risk of embryo loss must be considered.
• 38-40 years: Aneuploidy rate is about 40%-50%; the benefit of PGT-A is significant and should be combined with genetic counseling.
• >40 years: Aneuploidy rate >60%; PGT-A can improve transfer efficiency, but the number of retrieved oocytes and usable embryos decreases, requiring psychological preparation.

9. Special Situation Management

The following situations require specific testing strategies and genetic counseling:

• Cryptic translocation carriers: Normal conventional karyotype analysis but PGT-SR detects chromosomal copy number abnormalities. In such cases, additional SNP array or whole-genome sequencing is needed.
• Mitochondrial diseases: Neither PGT-A nor PGT-SR can detect mitochondrial mutations; specialized mitochondrial gene testing (extended application of PGT-M) is required.
• Robertsonian translocation carriers: PGT-SR can select normal or balanced embryos, but distinguishing between 1/3 normal karyotypes and 2/3 balanced karyotypes makes genetic counseling complex.
• Previous abnormal PGT result but still have transferable embryos: It is recommended to make decisions based on multiple factors including embryo morphological grading, mosaicism ratio, and patient preference; re-biopsy or repeat testing may be considered if necessary.