TAPS (Twin Anemia–Polycythemia Sequence): Antenatal Diagnosis by ΔMCA-PSV and Staging
Diagnostic Criteria for TAPS
Classical ultrasound criteria for antenatal suspicion of TAPS have a sensitivity of 46% and specificity of 100% for postnatally confirmed TAPS; positive and negative predictive values are 100% and 70%, respectively (ISUOG, 2025).
One of the most frequently used criteria for suspicion of TAPS in the prenatal period is MCA-PSV < 1 MoM in the donor and > 1.5 MoM in the recipient. Another common criterion is the intertwin difference of MCA-PSV > 0.5 MoM.
Updated Delphi Consensus Criteria
It has been demonstrated that a recipient with MCA-PSV > 1.0 MoM may remain polycythemic at birth; therefore, alternative diagnostic criteria were proposed. To achieve standardization, a Delphi consensus was convened, and the expert panel agreed on the following thresholds for antenatal diagnosis of TAPS (ISUOG, 2025):
| Parameter | Threshold |
|---|---|
| MCA-PSV in donor | > 1.5 MoM |
| MCA-PSV in recipient | < 0.8 MoM |
| Delta MCA-PSV between twins | > 1.0 MoM |
For diagnosis, the combination of MCA-PSV > 1.5 MoM in the donor and < 0.8 MoM in the recipient is sufficient, or isolated intertwin difference of delta MCA-PSV > 1.0 MoM.
Post-laser TAPS
When TAPS develops after laser coagulation for TTTS, role reversal must be considered: the former recipient may become anemic, and the former donor may become polycythemic (UOG, 2024). Early reports described significant changes in MCA-PSV in both twins (< 0.8 MoM in one and > 1.5 MoM in the other) in a subset of monochorionic diamniotic twins following laser surgery for TTTS.
Role in Staging of Complicated Twin Pregnancies
TAPS is classified as a complication that determines management strategy: the presence of TAPS is included in the criteria for complicated Stage-I TTTS (along with amniotic fluid pocket > 10 cm, preterm contractions, or short cervix < 2.5 cm), in which laser surgery is performed (UOG, 2024). The Quintero classification system is used for TTTS staging.
Frequently asked questions
What MCA-PSV thresholds should be used according to the current consensus?
According to the Delphi consensus (ISUOG, 2025): MCA-PSV > 1.5 MoM in the donor and < 0.8 MoM in the recipient, or delta MCA-PSV between twins > 1.0 MoM.
How do the new criteria differ from previous ones?
Previously, MCA-PSV < 1 MoM in the donor and > 1.5 MoM in the recipient, or intertwin difference > 0.5 MoM were used. The threshold for the recipient has been lowered to < 0.8 MoM, and delta has been increased to > 1.0 MoM, because a recipient with MCA-PSV > 1.0 MoM may remain polycythemic.
What is the diagnostic accuracy of ultrasound criteria for TAPS?
Sensitivity 46%, specificity 100%; positive and negative predictive values 100% and 70%, respectively, for postnatally confirmed TAPS (ISUOG, 2025).
What should be considered when TAPS develops after laser therapy for TTTS?
Role reversal is possible: the former recipient becomes anemic, and the former donor becomes polycythemic, which changes the interpretation of MCA-PSV.
How does TAPS affect TTTS classification?
The presence of TAPS classifies the case as complicated Stage-I TTTS, which indicates laser surgery; TTTS staging is performed according to the Quintero system.