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. 2021 Aug 5;225(6):593–599.e2. doi: 10.1016/j.ajog.2021.07.029

Table 2.

Definition specifics of placental infection with SARS-CoV-2 in order of rigor

Definite: documentation of viral presence, location in the placenta tissues, and replication, by:
  • 1)

    RNA probe to the antisense strand, showing a positive signal in placental tissues with appropriate positive and negative controls (validated reagents and sample protocols are detailed in Appendix A)

or
  • 2)

    Positive staining by RNA-ISH for double-stranded RNA, produced as replication intermediate for positive-sense RNA virus in placental tissues with appropriate positive and negative controls (validated reagents and sample protocols are detailed in Appendix A)

Probable: documentation of viral proteins or RNA within placental tissues, without evidence of active replication via:
  • 1)
    RNA probe to the positive-sense strand, showing a positive signal in placental tissues with appropriate positive and negative controls (Figure 3 ).

    Figure 3.

    Figure 3

    RNA-ISH positive signal in syncytial and cytotrophoblast (10× original)
    RNA-ISH, RNA in situ hybridization.
    Roberts et al. Consensus definition of SARS-CoV-2 placental infection. Am J Obstet Gynecol 2021.
or
  • 2)
    Positive staining by immunohistochemistry in placental tissues with antibodies directed to viral proteins, with appropriate positive and negative controls (validated reagents and sample protocols are detailed in Appendix B) (Figure 4 ).

    Figure 4.

    Figure 4

    Antispike protein IHC with signal in syncytial and cytotrophoblast (20× original)
    IHC, immunohistochemistry.
    Roberts et al. Consensus definition of SARS-CoV-2 placental infection. Am J Obstet Gynecol 2021.
Possible: less specific detection of virus. These approaches could be detecting viral particles engulfed by macrophages rather than actively replicating virus. RT-PCR of placental homogenates theoretically may have a positive result owing to maternal viremia (although this is a rare entity), rather than placental involvement.
  • 1)

    RT-PCR detection or quantification of viral RNA in PBS-rinsed placental homogenates, no tissue localization (validated reagents and sample protocols are summarized in Appendix C).

Note that an alternative approach is a 2-step approach, in which RT-PCR is used as a screen and then followed up with one of the methods recommended to confirm “definite” or “probable” infection. This hybrid/2-step approach would be more rigorous than RT-PCR alone and potentially more sensitive than the “definite” and “probable” approaches.
  • 2)

    Electron microscopic detection of viral-like particles in placental tissues.

Unlikely: Negative results from any of the above tests
No testing: placenta not tested

PBS, phosphate-buffered saline; RNA-ISH, RNA in situ hybridization; RT-PCR, reverse transcription polymerase chain reaction.

Roberts et al. Consensus definition of SARS-CoV-2 placental infection. Am J Obstet Gynecol 2021.