Pathways Case Studies: November 2023

A 69-year-old man from Wisconsin passed away unexpectedly at home in August. Three weeks prior, he presented with symptoms including hot flashes, sweating, shortness of breath, cough, blurry vision, right eye pain, epigastric pain, right shoulder pain, ankle swelling, redness, a swollen neck lymph node, and loss of balance. He also reported multiple bug bites. His medical history includes hypertension, hyperlipidemia, and type 2 diabetes mellitus. Autopsy findings are shown in the following photos.

Figure 1: Cross section of the left and right ventricles at mid-level.
Figure 2: Histologic section of the left ventricle (H&E, 5.0/40x).
Figure 3: Histologic section of the left ventricle (H&E, 40/40x).
Figure 4: Histologic section of the ventricular septum (H&E, 5.0/40x).
Figure 5: Histologic section of the right ventricle (H&E, 5.0/40x).
Figure 6: Histologic section of the atrioventricular node (H&E, 5.0/40x).

What is the most likely diagnosis?

  • Acute myocardial infarction
  • Lyme carditis
  • Arrhythmogenic cardiomyopathy
  • Hypersensitivity myocarditis

The correct answer is ...

Lyme carditis.

This decedent’s gross autopsy showed spotty areas of fibrosis in the heart. Histology showed carditis characterized by interstitial and perivascular lymphoplasmacytic infiltrates with background fibrosis that formed an intersecting curvilinear pattern (“road-map” distribution). The inflammatory infiltrates spanned across the left ventricle, ventricular septum, right ventricle, and the conduction system. Notably, his Lyme antibody modified two-tier test (both IgG and IgM) returned positive. These findings, along with clinical symptoms and geographic location, were consistent with Lyme carditis.

Lyme disease is the most common tick-borne infection in the United States, with an annual report of about 30,000 cases. The spirochetes of Borrelia burgdorferi sensu lato complex are transmitted by ticks (Ixodes ricinus complex), with the peak infection in late spring and summer. Clinical histories can be nonspecific, including viral-like symptoms, malaise, muscle and joint pains, lymphadenopathy, and shortness of breath. Erythema migrans (EM) rash occurs in 70%–80% of patients during the early stage. In the early disseminated phase, cardiovascular symptoms, including atrioventricular block, can occur in about 1.1% of Lyme infection, while fatal myocarditis is rare.

The diagnosis of Lyme carditis is based on clinical history, histology, and serology. Typical autopsy findings include pancarditis with interstitial and perivascular infiltrates, with limited myocyte necrosis. A key differential to hypersensitivity myocarditis is the lack of prominent eosinophilic infiltrates. Warthin-Starry stain (WS), a silver nitrate-based method, is helpful to identify spirochetes, but may have limited sensitivity and specificity due to the low load of the spirochetes and high background staining. Other methods include Borrelia burgdorferi immunohistochemistry (IHC) and nuclear acid detection using polymerase chain reaction (PCR). 

Serology testing for Lyme disease includes standard two-tier testing algorism (STTT) and modified two-tier testing algorism (MTTT). For MTTT, enzyme immunoassay (EIA) for total antibody testing against VlsE1 and pepC10 antigens is performed first. If positive, whole cell sonicate (WCS) IgG and IgM testing is performed to confirm the infection. In patients without past Lyme disease, MTTT has been reported to have comparable specificity (≥99%) to STTT, and close to 100% sensitivity for patients with carditis, neuroborreliosis, or arthritis. 

It is reasonable to consider arrhythmogenic cardiomyopathy as a potential differential diagnosis for sudden cardiac death. However, the typically gross findings would include fatty or fibrofatty infiltration of the right ventricle with chamber dilation, and histology would show transmural fibrofatty replacement of the myocytes. Given the medical history of hypertension, hyperlipidemia, and diabetes, acute myocardial infarction can be suspected. Autopsy for early infarct may be negative, or demonstrate pale discoloration, hemorrhage, and myocardium softening. Neither of the two scenarios would have pancarditis with extensive lymphoplasmacytic infiltrates.


  1. Muehlenbachs A, Bollweg BC, Schulz TJ, Forrester JD, DeLeon Carnes M, Molins C, Ray GS, Cummings PM, Ritter JM, Blau DM, Andrew TA, Prial M, Ng DL, Prahlow JA, Sanders JH, Shieh WJ, Paddock CD, Schriefer ME, Mead P, Zaki SR. Cardiac tropism of Borrelia burgdorferi: an autopsy study of sudden cardiac death associated with Lyme carditis. Am J Pathol. 2016 May;186(5):1195-205. 
  2. Sfeir MM, Meece JK, Theel ES, Granger D, Fritsche TR, Steere AC, Branda JA. Multicenter clinical evaluation of modified two-tiered testing algorithms for Lyme disease using Zeus Scientific commercial assays. J Clin Microbiol. 2022 May 18;60(5):e0252821.

Yi Zhu, M.D., Ph.D.

Resident, Anatomic Neuropathology
Mayo Clinic

Ross Zumwalt, M.D.

Senior Associate Consultant, Anatomic Pathology
Mayo Clinic

In your capacity as the lead physician of a healthcare facility, your team has grappled with enhancing the patient journey experience during laboratory visits. To address this, your data science unit devised a machine-learning (ML) model that can potentially identify patients who might go through unsatisfactory experiences during their visit to the lab. Your research team describes their model to be at the level of proof of concept, with a clinical research use case, and real-world representative data, the data pipeline is a prototype–caliber pipeline, and they identified users from a clinical user experienced perspective. Yet, they haven’t started stakeholder analysis or product caliber specifications. The team provided you with a model card to communicate their work so far. 

Figure 1

Contemplating how ready this ML model is for the real world, on a scale from 0 to 9, what is the Machine Learning Technology Readiness Level (MLTRL)? 

  • MLTRL1
  • MLTRL4
  • MLTRL6
  • MLTRL9

The correct answer is ...


In this vignette, we introduce two central concepts: the AI lifecycle framework and model cards. These concepts are pivotal for evaluating the maturity of a Machine Learning (ML) system. Our vignette focuses on an ML system devised within a multifaceted healthcare system, with anticipation of its integration into the prevailing software landscape. To ascertain the maturity of this AI technology, exploring the Machine Learning Technology Readiness Level (MLTRL) framework may be warranted. This framework considers the obligations of an ML system and emphasizes other crucial aspects like fairness, ethics, reliability, and robustness.

Drawing from experiences with ML systems across diverse sectors — including aerospace, defense, and civil engineering — Lavin, et al. delineated processes, and testing standards. This has enabled a structured approach for assessing the maturity of ML systems suited for real-world applications.

The objective of the MLTRL framework is to cultivate and launch ML systems that are robust, reliable, and responsible. This framework streamlines workflows and bolsters communication across teams at various AI system development stages. Its design mirrors the stages of AI product evolution, spanning research and development, productization, and deployment.

For instance, MLTRL4 serves as a proof of concept, showcasing the model's real-world data performance. However, at this juncture, the ML system hasn't advanced sufficiently to align with the requirements of data governance, product standards, code integrity, and regulatory compliance, as seen in MLTRL5,6. On the other hand, MLTRL1 denotes goal-oriented research, marking the nascent stages of ML model development. MLTRL9 signifies the culmination of this framework, where an ML system's deployment is continually monitored and refined.

Our vignette also underscores the importance of the MLTRL model card within the MLTRL framework. These model cards document and elucidate the performance attributes of an ML model, making them vital for both individual and functional communication. They serve to relay project updates and facilitate team member onboarding. Beyond this, they act as gauges for maturity, helping determine a model's position on the continuum between foundational research and deployment. By fostering communication across stakeholders and transcending the development phase, these cards distill some of the intrinsic knowledge embedded in the process. We propose that MLTRL cards act as foundational pillars supporting other tools, such as model cards, data cards, and ethical/responsibility parameters.

MLTRL model cards are cohesive and transferable documents, linking an ML system with its present milieu and its maturity journey. The external environment is portrayed via project data, overarching requirements, intended applications, and tacit knowledge. In contrast, the system's intrinsic attributes are conveyed through details on the model algorithm, testing status, potential biases, technical knowledge debt, and MLTRL stage debriefings. An intermediary layer exists between the internal and external domains, emphasizing data-related considerations like acquisition, sharing, privacy, and ethics.

The MLTRL model cards aren't merely standalone documents but community resources complementing tools like pivotal for documentation and data provenance. For instance, our choice was to adopt datasheets for datasets proposed by Google, given their transparency and ease of implementation. Additionally, MLTRL model cards can be synchronized with data cards using DVC (data version control), reinforcing best practices in ML system development.

Lastly, MLTRL model cards champion transparency and thoroughness, especially in AI clinical trials. Researchers can harness these cards to craft clinical trial protocols, aligning seamlessly with frameworks like SPIRIT-AI and CONSORT-AI. Moreover, these model cards pinpoint potential biases within an ML system, an aspect not inherently addressed in AI clinical trials. Effective reporting in AI trials commences with comprehensive documentation of ML system development. SPIRIT-AI clinical trial protocols, for example, can be augmented using TRL cards, enriching sections like title, background, interventions, and potential harms. 


  1. Staggers N, Gassert CA, Skiba DJ. Health professionals' views of informatics education: findings from the AMIA 1999 spring conference. J Am Med Inform Assoc. 2000;7(6):550-558. doi:10.1136/jamia.2000.0070550.
  2. A comparison of deep learning performance against health-care professionals in detecting diseases from medical imaging: a systematic review and medical analysis. The Lancet Digital Health, Volume 1, Issue 6, 2019. Pages e271-e297, ISSN 2589-7500
  3. Cruz Rivera S, Liu X, Chan AW, Denniston AK, and Calvert, MJ. (2020, September 9). Guidelines for clinical trial protocols for interventions involving artificial intelligence: the SPIRIT-AI extension. Nature News

EzzAddin Al Wahsh, M.D., M.B.A.

Fellow, Clinical Informatics
Mayo Clinic

Christopher Garcia, M.D.

Senior Associate Consultant, Computational Pathology and AI
Mayo Clinic

A 60-year-old man with a history of drug and alcohol abuse recently presented to the ER with active upper gastrointestinal bleeding, hypovolemic shock, and possible sepsis. The present liver biopsy was performed seven days after the ER presentation due to the persistently elevated liver enzymes, especially the alkaline phosphatase of over 400 units/L. 

Figure 1: H&E 4x
Figure 2: H&E 20x
Figure 3: Congo Red 20x
Figure 4: Trichrome 20x

What is your diagnosis?

  • Cardiac hepatopathy
  • Sinusoidal fibrosis
  • Light chain deposition disease
  • Amyloid deposition

The correct answer is ...

Amyloid deposition.

The biopsy shows liver parenchyma with acellular eosinophilic amorphous material deposition within the sinusoidal spaces with atrophy of the hepatocytes, characteristic of amyloidosis. Confirmatory Congo Red special stain demonstrated salmon-colored deposits, which showed apple green-birefringence under polarization, confirming amyloid deposits. The subsequent mass-spectrometry analysis identified the deposits to be AL-amyloid. 

Amyloidosis is a systemic disorder characterized by extracellular amyloid protein deposition commonly involving the liver.1 Autopsy studies have shown up to 95% of patients with amyloidosis to have hepatic involvement.2

Clinically, patients present with hepatomegaly and elevated serum alkaline phosphatase as the most common laboratory finding associated with hepatic amyloidosis.3

The five most common systemic forms of amyloid that involve the liver are amyloid light chain (AL), acquired amyloidosis (AA), B2 microglobulin amyloidosis (Abeta2M), and the two types of transthyretin amyloidosis (ATTR) (hereditary and non-hereditary). The present subtype, AL-amyloid, is associated with plasma cell dyscrasias, multiple myeloma, B-cell lymphoma, or Waldenstrom disease. The precursor proteins can be Kappa or Lambda immunoglobulin light chains derived from plasma cells. 

Amyloid deposits in the liver show similar morphologic findings as other organ sites, as it presents as an acellular eosinophilic amorphous material, which may deposit in the sinusoids, portal tracts, vessel walls, or any mix of the mentioned sites. 

Specific subtypes of amyloid may show different patterns of deposition within the liver. For example, some studies have demonstrated that AA amyloidosis shows deposits primarily in the blood vessels of the portal tracts, whereas AL amyloidosis usually shows a “sinusoidal” pattern. However, the distribution pattern alone is not used clinically to subtype amyloid.4 Additionally, the deposit morphology may also help determine the subtypes of amyloid deposits as large round eosinophilic globular deposits within the hepatocytes or reticuloendothelial cells characterize the “globular amyloid deposits,” which are highly specific for leukocyte chemotactic factor (LECT2)-associated amyloidosis.5

Previously, immunostains were utilized to subtype the amyloid. However, due to their challenges in interpretation, laser microdissection with mass spectrometry is now the preferred method that is both sensitive and specific for identifying all amyloid subtypes.6

The differential diagnosis for hepatic amyloid is the presence of other extracellular deposits that can look similar on H&E, such as dense collagen deposits, collection of red blood cells, or light chain deposition. The application of Congo Red Trichrome stain can be helpful for their distinction. 

The Light chain deposition disease is a rare monoclonal plasma cell proliferative disorder characterized by the deposition of light chains in the basement membranes. It primarily affects the kidneys, and the liver is the most common site of extrarenal involvement. These typically deposit within the sinusoidal spaces similar to amyloid but are Congo Red stain negative.7


  1. Torbenson M, Moreira R, and Zhang, L. Surgical Pathology of the Liver. (Wolters Kluwer Health, 2017).
  2. Iwata T, Hoshii Y, Kawano H, et al. Hepatic amyloidosis in Japan: histological and morphometric analysis based on amyloid proteins. Hum Pathol. 1995;26(10):1148-1153. doi:10.1016/0046-8177(95)90279-1
  3. Park MA, Mueller PS, Kyle RA, Larson DR, Plevak MF, Gertz MA. Primary (AL) hepatic amyloidosis: clinical features and natural history in 98 patients. Medicine (Baltimore). 2003;82(5):291-298. doi:10.1097/
  4. Looi LM, Sumithran E. Morphologic differences in the pattern of liver infiltration between systemic AL and AA amyloidosis. Hum Pathol. 1988;19(6):732-735. doi:10.1016/s0046-8177(88)80181-3
  5. Chandan VS, Shah SS, Lam-Himlin DM, et al. Globular hepatic amyloid is highly sensitive and specific for LECT2 amyloidosis. Am J Surg Pathol. 2015;39(4):558-564. doi:10.1097/PAS.0000000000000373
  6. Vrana JA, Gamez JD, Madden BJ, Theis JD, Bergen HR 3rd, Dogan A. Classification of amyloidosis by laser microdissection and mass spectrometry-based proteomic analysis in clinical biopsy specimens. Blood. 2009;114(24):4957-4959. doi:10.1182/blood-2009-07-230722
  7. Cristino A, Pais C, Silva R, Carrola P. Light-Chain Deposition Disease with Prominent Hepatic Involvement. Eur J Case Rep Intern Med. 2017;4(3):000545. Published 2017 Apr 27. doi:10.12890/2017_000545

Byoung Uk Park, M.D.

Fellow, Gastrointestinal & Liver Pathology
Mayo Clinic

Photo of Chady Meroueh, M.D.

Chady Meroueh, M.D.

Consultant, Anatomic Pathology
Mayo Clinic
Assistant Professor of Laboratory Medicine and Pathology
Mayo Clinic College of Medicine and Science

A 17-year-old girl presents to her primary care physician with episodic watery diarrhea, nausea, and crampy abdominal pains. She is up to date on her vaccinations and denies any recent travels or changes in her diet. Family history and past medical history are both unremarkable; moreover, physical exam is only significant for mild, diffuse abdominal tenderness. Upper GI endoscopy demonstrates diffuse nodular mucosa in the fundus. A fundic biopsy is taken and shown below.

Figure 1: Upper GI Endoscopy
Figure 2: Stomach biopsy

Which of the following is most characteristic of her condition?

  • Anemia and upper gastrointestinal bleeding symptoms are more common in younger patients.
  • Definitive therapy is a combination of PPI and antibiotics.
  • Biopsy of nodules on endoscopy will demonstrate diagnostic pathology.
  • The subepithelial space will stain with Congo red.

The correct answer is ...

Anemia and upper gastrointestinal bleeding symptoms are more common in younger patients.

Collagenous gastritis is described in the literature has having a bimodal distribution: a pediatric subtype and an adult subtype. The pediatric variant of the condition is more likely to present with anemia and upper GI bleeds. The bleeding episodes are thought to be due to entrapment of blood vessels within the subepithelial collagen plate (please review the biopsy image and note the many vessels within said collagen). The adult form of the condition, while less likely to be associated with anemia/UGIBs, often presents within the setting of other comorbidities, such as lymphocytic gastritis, celiac sprue, autoimmune disorders (e.g., Hashimoto thyroiditis, polymyositis), as well as collagenous colitis.1 There has also been a case report of collagenous gastritis in a patient with systemic lupus erythematosus, but more research needs to be done to conclusively determine whether or not there is an association between the two clinical entities.2 It is also worth noting that this dichotomy is merely a trend and not definitive; for instance, anemia and bleeding symptoms have been reported in some adult patients as well, though it appears to be a more common presentation in the younger population.

The subepithelial space will stain with Congo red.

While the subepithelial material may resemble amyloid deposits, it is important to allow the patient presentation and history to guide the diagnosis. While amyloidosis is certainly worth placing in the differential given the biopsy specimen, amyloidosis is much more common in the setting of other comorbidities. For instance, primary amyloidosis (AL amyloidosis) is typically secondary to the overproduction of immunoglobulin light chains in the setting of a plasma cell dyscrasia — such as multiple myeloma or Waldenström macroglobulinemia. Moreover, secondary amyloidosis (AA amyloidosis) is due to deposition of serum amyloid A protein. This is seen in chronic inflammatory conditions such as rheumatoid arthritis, as well as juvenile rheumatoid arthritis. Another class of amyloidosis worth mentioning involves the deposition of transthyretin within liver and cardiac tissue. These patients tend to present with symptoms of heart failure.3

Definitive therapy is a combination of PPI and antibiotics.

To date, no definitive therapy has been agreed upon. An important concept to keep in mind for this entire case (and any case for that matter) is that, while we learn textbook cases, we generally do not live textbook lives. Still, we hope that our medical tomes might at least guide us towards piecing together some clinicopathologic gestalt. But what should we do when the textbook is still being written? Collagenous gastritis, as a clinical entity, was first described in 1989 by Colletti and Trainer.As such, this condition is relatively new in its description in the literature; consequently, there is currently no definitive cure or treatment for this condition. Varying therapies — such as PPIs, H2 blockers, sucralfate, misoprostol, 5-aminosalicylates, and corticosteroids — have been tried with variable success. Topical, targeted budesonide administration has shown promising results for managing symptoms; that said, a standard of care has yet to be definitively defined for collagenous gastritis.5 Antibiotics are only used when there is an underlying infectious etiology, which is not the case in majority of reported cases.

Biopsy of nodules on endoscopy will demonstrate diagnostic pathology.

This may seem counterintuitive, but the raised, polyp-like areas of nodularity actually demonstrate normal stomach histology. It is actually the areas of depression around these nodules which are best for biopsy, as these depressed areas evince subepithelial collagenous thickening. Think of it another way: the nodular areas are normal histologically but appear more salient grossly because they are surrounded by areas of collagen deposition.6

Other information regarding collagenous gastritis:

This is a very rare condition. There may be around 300 cases described worldwide, though this is a somewhat liberal estimate, with some experts claiming even fewer cases globally. So far, there does not appear to be a geographic preference. Additionally, some studies suggest there may be a slight female preponderance, though other studies have reported no gender preference. Presenting symptoms may include nausea, vomiting, recurrent crampy abdominal pain, and watery diarrhea. To date, there have been no reported cases of dysplasia or carcinoma arising from or as a consequence of collagenous gastritis. The pathogenesis is unclear, but it is thought that an inciting inflammatory process occurs, which results in subsequent collagen deposition.7


  1. Leung ST, Chandan VS, Murray JA, Wu TT. Collagenous Gastritis: Histopathologic Features and Associations with Other Gastrointestinal Diseases. Am J Surg Pathol. 2009; 33: 788-798.
  2. Al-Kandari A, Al-Alardati H, Sayadi H, Al-Jubaibi B, Mawardi M. An unusual case of collagenous gastritis in a middle- aged woman with system lupus erythematosus: a case report. J Med Case Reports. 2014; 8:278.
  3. Muchtar E, Dispenzieri A, Magen H, Grogan M, Mauermann M, McPhail ED, Kurtin PJ, Leung N, Buadi FK, Dingli D, Kumar SK, Gertz MA. Systemic amyloidosis from A (AA) to T (ATTR): a review. J Intern Med. 2021 Mar;289(3):268-292.
  4. Colletti RB & Trainer TD. Collagenous gastritis. Gastroenterology. 1989; 97(6): 1552-1555.
  5. Choung RS, Sharma A, Chedid VG, Absah I, Chen ZE, Murray JA. Collagenous gastritis: Characteristics and Response to Topical Budesonide. Clin Gastroenterology and Hepatology. 2022;
  6. Jin X, Koike T, Chiba T, Kondo Y, Ara N, Uno K, et al. Collagenous gastritis. Digestive Endoscopy. 2013; 25: 547-549.
  7. Jain R & Chetty R. Collagenous gastritis. Int J Surg Path. 2010; 18(6): 534-536.

Dustin Parsons, M.D. 

Resident, Anatomic & Clinical Pathology
Mayo Clinic

Eric Chen, M.D., Ph.D.

Consultant, Anatomic Pathology
Mayo Clinic
Associate Professor of Laboratory Medicine and Pathology
Mayo Clinic College of Medicine and Science

A 72-year-old woman presents with vaginal bleeding and is subsequently diagnosed with endometrioid adenocarcinoma (FIGO grade 2/3) (Figure 1). Immunohistochemistry (IHC) for mismatch repair (MMR) proteins is notable for an absence of MLH1, PMS2, MSH2, and MSH6 expression (Figure 2). 

Figure 1
Figure 2

What underlying molecular genetic results may adequately account for this rare immunophenotypic pattern?

  • Failure of IHC positive control
  • POLE alteration (variant of undetermined significance, VUS) and PMS2 pathogenic alteration
  • MLH1 promoter hypermethylation and MSH2 pathogenic alteration (c.560T>C, p.Leu187Pro)
  • MLH1 promoter hypermethylation and MSH6 alteration (variant of undetermined significance, VUS)

The correct answer is ...

MLH1 promoter hypermethylation and MSH2 pathogenic alteration (c.560T>C, p.Leu187Pro).

MLH1 promoter hypermethylation leads to silencing of MLH1 gene expression thus preventing subsequent protein formation. Consistent with the patient’s age, this form of epigenetic inactivation occurs most commonly in the context of sporadic (versus hereditary) tumor development. Loss of MLH1 would also account for the loss of PMS2 protein expression given the existence of the two proteins as a dimer. Independently, a pathogenic missense variant in MSH2 resulting from the instability created by MLH1/PMS2 loss would lead to the loss of MSH2, which would in turn lead to concomitant loss of MSH6 expression. 

POLE alteration (variant of undetermined significance) and PMS2 pathogenic alteration 

Although a POLE pathogenic alteration could, in theory, result in additional downstream alterations (including in the MMR genes) that could, in turn, result in loss of IHC staining for all MMR proteins. However, only a VUS was identified in POLE and the overall mutation spectrum of the tumor was not hypermutated. In addition, there is limited evidence available to support this theory. Although a PMS2 pathogenic alteration could explain the loss of PMS2 by IHC, not all PMS2 mutations cause the loss of MLH1. The PMS2 mutation would not explain the loss of MSH2/MSH6.

MLH1 promoter hypermethylation and MSH6 alteration (variant of undetermined significance, VUS)

MLH1 promoter hypermethylation would explain the loss of MLH1 and PMS2 by IHC. However, a VUS in MSH6 would not necessarily explain the loss of MSH6. Furthermore, MSH2 can be expressed by IHC in the absence of MSH6

Failure of IHC positive control

Although a failure of the positive control can certainly explain the loss of any protein by IHC, background staining of non-neoplastic cells (stroma/lymphocytes) in this case rules out this possibility (Figure 2).


  1. Nielsen SV, Stein A, Dinitzen AB, et al. Predicting the impact of Lynch syndrome-causing missense mutations from structural calculations. PLoS Genet. 2017;13(4):e1006739. Published 2017 Apr 19. doi:10.1371/journal.pgen.1006739
Mazen Atiq MBBS portrait square

Mazen Atiq, M.B.B.S.

Fellow, Molecular Genetic Pathology
Mayo Clinic

Kevin Halling, M.D., Ph.D.

Consultant, Laboratory Genetics and Genomics
Mayo Clinic
Professor of Laboratory Medicine and Pathology
Mayo Clinic College of Medicine and Science

Ande Rumilla, M.D.

Consultant, Laboratory Genetics and Genomics
Mayo Clinic
Assistant Professor of Laboratory Medicine and Pathology
Mayo Clinic College of Medicine and Science

A 25-year-old woman with a twin gestation complicated by twin-to-twin transfusion syndrome underwent an amniocentesis procedure, and amniotic fluid from Twin A was obtained. There were no other anomalies or complications noted with the pregnancy. A chromosomal microarray study performed on cultured cells from the amniotic fluid showed mosaic trisomy 6 as displayed on the figure below. Maternal cell contamination was ruled out. 

Figure 1

Given this chromosomal microarray result, what is the most appropriate interpretation of this finding, and what subsequent action (if any) should be taken?

  • The microarray result is consistent with mosaic trisomy 6 in the fetus, which is incompatible with life, and no further action is needed.
  • The microarray result most likely represents a sample mix-up, as mosaic trisomy 6 is incompatible with life. A new amniotic fluid sample should be requested.
  • The microarray result represents pseudo-mosaicism due to culture artifact. As mosaic trisomy 6 is incompatible with life, no further action is needed.
  • The microarray result may represent pseudo-mosaicism due to culture artifact. Additional cytogenetic studies should be performed on an independent culture to confirm or refute this finding.

The correct answer is ...

The microarray result may represent pseudo-mosaicism due to culture artifact. Additional cytogenetic studies should be performed on an independent culture to confirm or refute this finding.

Mosaicism is defined as the presence of two or more populations of cells with differing genetic complements. When a mosaic finding is observed in prenatal specimens, several possibilities are important to consider when assessing if this represents true fetal mosaicism. In prenatal results from cultured amniotic fluid cells, apparent mosaicism may represent true fetal mosaicism, maternal cell contamination, or pseudomosaicism due to culture artifact. Since decisions regarding termination or continuation of a pregnancy can be influenced by these results, it is imperative to determine whether true fetal mosaicism is present or not. 

In this case, since maternal cell contamination has been ruled out, and low-level mosaicism could be present in a fetus with no obvious abnormalities by ultrasound, additional studies are required (karyotype, fluorescence in situ hybridization, or repeat microarray studies) to determine if true fetal mosaicism is present. These studies can either be performed on residual uncultured amniotic fluid specimen (if available) or on independent cultures derived from the same amniotic fluid sample that was received. If true fetal mosaicism is present, trisomy 6 cells should be detected in two or more independent cultures. If pseudomosaicism due to culture artifact is the cause of the mosaic trisomy 6, then the abnormality will only be present in the culture that was used for the chromosomal microarray study.


  1. Wegner RD, Entezami M, Knoll U, Horn D, Sohl S, Becker R. Prenatal diagnosis of fetal trisomy 6 mosaicism and phenotype of the affected newborn. Am J Med Genet A. 2004 Jan 1;124A(1):85-8. doi:10.1002/ajmg.a.20407. PMID: 14679592.
  2. Destree A, Fourneau C, Dugauquier C, Rombout S, Sartenaer D, Gillerot Y. Prenatal diagnosis of trisomy 6 mosaicism. Prenat Diagn. 2005 May;25(5):354-7. doi:10.1002/pd.1149. PMID: 15906424.
  3. Cockwell AE, Baker SJ, Connarty M, Moore IE, Crolla JA. Mosaic trisomy 6 and maternal uniparental disomy 6 in a 23-week gestation fetus with atrioventricular septal defect. Am J Med Genet A. 2006 Mar 15;140(6):624-7. doi:10.1002/ajmg.a.31129. PMID: 16470696.
  4. McKinlay Gardner, R. J, and others, 'Chromosome Abnormalities Detected at Prenatal Diagnosis', Gardner and Sutherland's Chromosome Abnormalities and Genetic Counseling, 5 edn, Oxford Monographs on Medical Genetics (New York, 2018; online edn, Oxford Academic, 1 Feb. 2018),

Sharri Cyrus, M.B.B.S.

Fellow, Laboratory Genetics and Genomics
Mayo Clinic

Nicole Hoppman, Ph.D.

Consultant, Laboratory Genetics and Genomics
Mayo Clinic
Associate Professor of Laboratory Medicine and Pathology
Mayo Clinic College of Medicine and Science

MCL Education

This post was developed by our Education and Technical Publications Team.