Despite this attempt to standardize reporting of ACR, variability occurred in the interpretation of histologic grading among pathologists. In 2001, the Banff Allograft Pathology Group invited pathologists, cardiologists, and cardiac surgeons to discuss their experiences after a decade of using the 1990 ISHLT system. In 2004, under the direction of the ISHLT, a working group composed of an international multidisciplinary team of subspecialists in cardiac transplantation met to review the ISHLT 1990 definitions.

A major controversy of the 1990 schema was the diagnosis and treatment of grade 2 rejection [12, 13]. While many transplant centers used grade 2 rejection as the tipping point for treatment, others observed that, in the majority of cases, grade 2 lesions resolved without treatment. This and other considerations led the working group to propose a revised schema (◘ Table 28.2, ◘ Figs. 28.1, 28.2, and 28.3) designated by the suffix “R.” In the revised classification system, the 1990 grade 2 rejection was categorized as mild rejection (ISHLT 2004 grade 1R). Additionally, 1990 grades 3A and 3B were combined into grade 2R which represents moderate-/intermediate-grade ACR.

Despite the revised grading system, several pitfalls remain in the diagnosis of ACR. One challenge is accurate recognition of myocyte damage. The morphologic spectrum of myocyte damage is wide. These lesions may be subtle and represented by vacuolization of myocytes, perinuclear halo, ruffling of the cytoplasmic membrane, and irregular myocyte border. Other changes are more easily recognizable, such as outright splitting or branching of myocytes and myocyte encroachment with partial disruption by inflammatory infiltrate [4, 14]. In the revised ISHLT scheme, myocyte damage is described as “clearing of sarcoplasm and nuclei with nuclear enlargement and occasionally prominent nucleoli” [6]. Architectural distortions and myocyte dropout also frequently indicate myocyte damage. These changes are subtle, open to interpretation, and account for major interobserver variability in grading ACR.

Another pitfall in accurately diagnosing ACR is the difficulty in distinguishing histological features of Quilty lesions from true ACR lesions. Quilty lesions are dense endocardial lymphocytic infiltrates composed of predominantly T lymphocytes with admixed B cells, occasional macrophage, and plasma cells that can also be seen in the endocardium of transplanted hearts. Quilty infiltrates can either be confined to the subendocardial regions (previously known as Quilty A) or infiltrate deeply into the myocardium (previously known as Quilty B). Infiltrating Quilty lesions can be big. A biopsy without overlaying the subendocardial component of Quilty can be easily mistaken for moderate rejection. In such instances, it’s helpful to examine multiple levels and carefully assess the biopsies to identify the continuity of the lesion from the myocardium to the overlying subendocardial component. Regulated on Activation, Normal T Cell Expressed and Secreted (RANTES)-positive cells are abundant in acute cellular rejection [15], and an immunohistochemical stain for RANTES may help differentiate true rejection lesions from Quilty infiltrates.

Ischemic lesions can be associated with severe rejection or can be secondary to a prolonged ischemia time [16]. In the revised grading system, ischemia is divided into early ischemia, which occurs within 6 weeks of transplantation, and late ischemic injury. Late ischemic injury may explain cardiac allograft dysfunction secondary to severe allograft atherosclerosis and should be reported. Early posttransplant ischemia is subendocardial with foci of myocyte necrosis with or without associated macrophages and polymorphonuclear leukocytes. Distinguishing the healing phase of ischemic injury from moderate rejection may be difficult. In such situations, clinical correlation and communication among the cardiologists are helpful in management of the patient and in determining the patient’s status and best treatment options.

Previous biopsy sites are very commonly found during transplant surveillance biopsies [17]. Due to inherent anatomic configuration of the allograft, the bioptome is guided to the same general region at the interventricular septum, leading to sampling of previous biopsy sites. Healing biopsy sites, especially when cut tangentially, may have entrapped myocytes with associated inflammation and are sometimes difficult to distinguish from ACR.

Because of improved immunosuppressive regimens, posttransplant lymphoproliferative disease (PTLD) is rare in contemporary transplant centers. In the rare cases where PTLD does occur, it can be seen on routine surveillance EMB. Distinguishing PTLD from ACR is very important as the former is managed by decreasing immunosuppressants and the latter by increasing them. Decreasing immunosuppression may lead to complete regression of PTLD [18, 19]. The majority of PTLD cases seen today are large B-cell lymphomas. Immunohistochemical stains for T and B cells and in situ hybridization for detection of Epstein-Barr virus (EBV) are performed for making the diagnosis. T-cell lymphomas can also occur, but usually present in the extranodal sites [20].

Additional rare findings on EMB that should be carefully assessed include the presence of chordae tendineae, valvular tissue, adipose tissue, dystrophic calcifications, and intussusception of small arteries. Chordal rupture may or may not result in clinically significant tricuspid regurgitation [21–23]. The presence of chordae should be described in the report and correlated with clinical findings. Adipose tissue is mostly seen in the epicardial region. Microscopic foci of fat, however, may be present within the myocardium in all four chambers, especially in obese patients, older patients, and patients taking steroid hormones. On the rare occasion that the bioptome may actually sample the right ventricular free wall, a finding of adipose tissue may suggest perforation. The presence of mesothelial cells associated with adipose tissue suggests an epicardial sampling. Tamponade is rare after such events because organized pericarditis usually forms a dense, fibrous, protective layer around the myocardium, but the findings should be reported to the clinical team.