Screening both donors and recipients is essential in solid organ transplantation (SOT), and close collaboration between laboratory medicine experts and clinicians is fundamental to ensuring good patient outcomes. To estimate the risk of post-transplant infectious complications, clinicians perform a detailed history and physical exam, and review laboratory screening tests (1).
Clinicians and laboratorians work together to achieve five main goals of pre-transplant screening: identifying infections which may disqualify the recipient; facilitating transplants from donors infected with certain pathogens to recipients who are already infected with those agents (i.e., hepatitis C virus [HCV] or HIV); identifying and treating active pre-transplant infections; defining the risk of infection and determining strategies for preventing or mitigating post-transplant infections; and implementing prophylactic interventions, such as updating vaccination status. This mini-review focuses on laboratory screening tests performed during pre-transplant evaluations of asymptomatic transplant candidates (summarized in Table 1).
All SOT candidates should be evaluated for latent syphilis and TB (2, 3). Along these lines, a detailed social history is of utmost importance. Clinicians need to know SOT candidates’ country and city of origin, prior sites of residence, travel history, sick contacts, animal and environmental exposures, current and prior occupations, hobbies, TB risk factors, sexual habits, and substance abuse history. This information helps identify recipients at risk for Mycobacterium tuberculosis, parasites, and endemic fungi, as well as their risk for future infections. In the case of syphilis screening, it is important to obtain from local public health departments details of any prior testing and treatment.
There are two types of serologic tests for syphilis: non-treponemal tests and treponemal-specific tests. Non-treponemal tests detect antibodies against cardiolipin and are not specific. In contrast, treponemal tests detect antibodies against specific treponemal antigens, but traditionally have been more labor intensive. These characteristics have led to a two-step testing algorithm. Labs use non-treponemal assays (e.g., rapid plasma reagin or venereal disease research laboratory) to screen potential transplant candidates, then confirm reactive samples with a treponemal-specific test. The Centers for Disease Control and Prevention (CDC) specifically recommends the Treponema pallidum particle agglutination (TP-PA) assay because of its superior specificity and sensitivity compared with the fluorescent treponemal antibody absorption assay. Patients with a documented history of adequate treatment who have a positive non-treponemal test at low titer confirmed by TP-PA are considered serofast, do not harbor live organisms, and require no further interventions. Any patient with a positive non-treponemal test confirmed by TP-PA has an active infection and should be treated appropriately.
Because of their impaired immune response, transplant recipients are 20–74 times more likely to harbor TB than the general population, and their crude mortality is 20–30% (3). Active TB in SOT recipients may result from reactivation of latent infection in the recipient or donor tissue, unrecognized active disease in the donor or recipient at the time of transplantation, or de novo post-transplant infection.
A careful history in search of TB risk factors is a key component of pre-transplant evaluations. Epidemiologic risk factors include: close contact with a person who has active TB; birth, residence, or relief work in areas of the world with high rates of TB; history of injection drug use; and residence or work in institutional settings that pose an increased risk of TB exposure (correctional facilities, healthcare settings, and homeless shelters). Patients with evidence of prior TB based on chest imaging are also at risk for post-transplant reactivation.
All transplant candidates should be evaluated for latent TB infection. Detecting a cellular immune response against M. tuberculosis-specific antigens is the commonly accepted indirect measure of latent infection. The two methods to assess cellular immune response include the in vivo tuberculin skin test (TST) and ex vivo interferon gamma release assays (IGRA). The TST elicits a delayed–type hypersensitivity reaction after local intradermal application of purified protein derivative. A diameter of induration ≥5 mm measured 48–72 hours after inoculation is considered a positive result in HIV-infected patients, recent contacts of persons with infectious TB, patients with fibrotic changes on chest radiograph, and immunocompromised hosts, including transplant candidates. This 5 mm cut-off is lower than those used in other populations: ≥10 mm in those with other TB risk factors; ≥15 mm in individuals without any TB risk factors.
IGRAs measure T cell release of interferon gamma (IFN-γ) following stimulation with antigens present in M. tuberculosis. The two methods for quantifying IFN-γ production include: analyzing the supernatant of stimulated whole blood cells using enzyme-linked immunosorbent assay (ELISA) (QuantiFERON-TB Gold), and quantifying the number of T cells producing IFN-γ by enzyme-linked immunospot (T-SPOT.TB). A sample is considered positive if the measure exceeds a specific threshold relative to the controls. Patients who test positive should undergo a thorough evaluation to rule out active infection, and should be treated if they have no evidence of active TB and have not already had therapy for latent TB.
IGRAs have some advantages over TST, including higher specificity due to the use of antigens derived from M. tuberculosis that are absent in bacille Calmette-Guerin strains and most environmental mycobacteria. IGRAs also feature stimulation with positive and negative controls that helps discriminate true negatives from false negatives due to anergy or an immunosuppressive condition. A recent prospective study in South Korea revealed that four of 272 TST–negative renal transplant recipients developed post-transplant TB. All four had a positive T-SPOT.TB assay result. In addition, there were no cases of post-transplant reactivation in patients who tested negative or had an indeterminate result.
Although this data is encouraging, none of the assays is perfect. For example, a current negative test—especially in patients with a well-documented prior positive test—is likely the result of immunosuppression associated with organ failure. Particular attention should be paid to transplant candidates who have negative results but are at high risk for post-transplant TB. Many authorities recommend chemoprophylaxis for transplant candidates who have had close contact with a person with active TB or radiographic evidence of prior TB, even when their IGRA test results are negative.
Transplant recipients are at risk for several parasitic infections, including toxoplasmosis, strongyloidiasis, Chagas disease, and others.
Toxoplasmosis is an uncommon but highly morbid infection after transplantation. All potential heart transplant candidates should be screened for prior evidence of Toxoplasma infection by testing for the presence of specific IgG antibodies. The donor’s and recipient’s serologic status are used to stratify the risk of Toxoplasma reactivation. The highest risk of reactivation (50–75%) occurs in seronegative individuals who receive hearts from seropositive donors, for whom prophylaxis is indicated (4, 5).
Strongyloidiasis, caused by the helminth parasite Strongyloides stercoralis, affects 100 million people worldwide, mostly in tropical and subtropical areas. This parasite has the unique ability to complete its entire life cycle within one host, enabling it to persist for decades and cause clinical manifestations many years after the initial infection. Strongyloidiasis is a devastating disease in transplant recipients, with 50–70% mortality in those with hyperinfection syndrome and disseminated infection (4).
Post-transplant strongyloidiasis results from primary infection, reactivation of latent infection, or donor-derived infection. Epidemiologic risk factors for reactivation include a history of a parasitic infection, unexplained eosinophilia, and residence in or travel to an endemic area. Laboratories should screen SOT candidates with these risk factors for latent infection via serial stool ova and parasite examinations and Strongyloides IgG measured by ELISA.
Standard stool examination is notoriously insensitive for detecting strongyloidiasis infection because the larvae are intermittently excreted and most asymptomatic patients have a low parasite burden. The agar plate method has the highest sensitivity, and involves inoculating an agar plate with stool sample and incubating it for at least 2 days. As the larvae crawl, they carry bacteria with them, creating visible tracks on the agar surface. IgG measured by ELISA has a high sensitivity and specificity for diagnosing strongyloidiasis. False-positives are likely the result of cross-reactivity with other helminth infections.
Chagas disease is caused by the protozoan parasite Trypanosoma cruzi carried by triatomine bugs—known colloquially as conenose bugs, kissing bugs, and assassin bugs—that bite and transmit T. cruzi to mammals, including humans. Historically, transmission and morbidity were concentrated in rural areas of Latin America; however, during the past several decades, infected persons have migrated to the United States. A single-center, prospective cohort study from California that enrolled patients with a new diagnosis of non-ischemic cardiomyopathy with a history of previous residence in Latin America for at least 12 months found that 20% had Chagas disease (7).
Chagas disease in SOT plays out in three important scenarios. First, heart transplant candidates with chronic T. cruzi infection are at risk of post-transplant reactivation. Second, non-cardiac transplant recipients with chronic T. cruzi infection also are at risk for post-transplant reactivation. Third, uninfected SOT recipients who receive organs or blood from T. cruzi-infected donors (6).
Diagnosis of chronic infection requires serological methods to detect IgG antibodies to T. cruzi. Heart transplant candidates with non-ischemic cardiomyopathy and epidemiological risk factors for Chagas disease should be tested with two serological assays based on different antigens (e.g., whole parasite lysate and recombinant antigens) and/or techniques (e.g., ELISA plus either an immunofluorescent antibody assay or a radioimmune precipitation assay). No single assay has sufficient sensitivity and specificity to be relied upon solely. Studies from endemic areas suggest the rate of post-transplantation T. cruzi reactivation in heart transplant recipients with Chagas cardiomyopathy is 21–39%, while a single center study from the United States suggests a higher rate of 45% (6). Monitoring for T. cruzi reactivation via whole blood PCR, microscopy of blood buffy coat, and endomyocardial biopsies may avoid the fatal consequences of reactivation. Patients with clinical or laboratory evidence of reactivation should be treated with benznidazole.
Studies of renal transplant recipients with chronic Chagas infection in endemic areas report a rate of reactivation of 8–17% (4). Based on these data, it is reasonable to screen non-heart transplant candidates with epidemiologic risk factors for Chagas disease. Two questions should prompt screening for Chagas. First, was the transplant candidate born in or a resident for a significant time in Latin America? Second, was the recipient's mother born in Latin America? Transplant recipients with confirmed T. cruzi infection should be monitored for post-transplant Chagas disease reactivation via PCR and microscopic examination of the buffy coat. Those with clinical or laboratory evidence of reactivation should be treated with benznidazole.
Assays approved by the Food and Drug Administration (FDA) for blood donor screening are also recommended for SOT candidates. An FDA–cleared test with documented good performance characteristics is an alternative. Positive results should be confirmed by a recombinant antigen-based immunoblot or radioimmune precipitation assay. Transplant programs need to weigh the risks and benefits of using commercial assays as false negatives have been reported.
Selective screening for leishmaniasis, schistosomiasis, malaria, babesiosis, and other parasitic infections should be considered prior to transplant in patients with appropriate exposure risk (4).
Endemic Fungal Infections
Fungal infections in transplant recipients can have dire consequences. For example, the fungal infection Coccidioidomycosis, also called Valley fever, occurs in 1.4–6.9% of SOT recipients in endemic areas (8). Preventing post-transplant coccidioidomycosis is imperative since the mortality associated with this complication is high. Transplant candidates living in endemic areas are at risk for primary infection after organ transplantation, reactivation of previously acquired infection, and donor-derived infections.
There is no clear consensus among SOT programs in endemic areas about the optimal prophylactic strategy to prevent post-transplant coccidioidomycosis (8). Some programs favor universal prophylaxis while others use a targeted approach (10). In the latter, patients receive fluconazole if they have a prior history of coccidioidomycosis, recent or current infection, or positive serologic results. Studies performed in endemic areas provide some support for screening transplant candidates with Coccidioides-specific IgG and IgM by enzyme immunoassay and immunodiffusion and IgG by complement fixation for evidence of prior infection. In non-endemic areas, clinicians need to determine whether transplant candidates have a history—even in the distant past—of residing in an endemic area. Those who do should be screened with the same assays used in endemic areas.
Histoplasmosis and Blastomycosis
Post-transplant histoplasmosis is rare, with an estimated incidence <1%, even in endemic areas (8). A retrospective study from a hyper-endemic area that included 449 SOT recipients with a mean follow-up time >16 months found no cases of post-transplant histoplasmosis (11). The researchers did not detect a correlation between pre-transplant serologies and reactivation. Based on these data, the American Society of Transplantation Infectious Diseases Community of Practice does not recommend screening for serological evidence of prior infection with Histoplasma capsulatum in asymptomatic transplant candidates.
Blastomycosis is also quite rare. Data from the Transplant-Associated Infection Surveillance Network suggests that the 12-month cumulative incidence of blastomycosis is less than 1% (12). Current serological assays lack sensitivity and specificity as screening tools. Collectively these data do not support screening with serological assays for evidence of prior blastomycosis infection in asymptomatic transplant candidates.
A proper pre-transplant evaluation also includes screening for many viral infections, including HIV, hepatitis B virus (HBV), HCV, cytomegalovirus (CMV), Epstein-Barr Virus (EBV), and others.
Screening for HIV is required in pre-transplant evaluations. Though no longer an absolute contraindication, immunologic and virologic control must be documented prior to transplantation. Most recently, the HIV Organ Policy Equity Act allows for research about transplanting organs from HIV-positive donors into HIV-positive recipients. In this context, screening for HIV will also help allocate organs.
HBV and HCV
All transplant candidates should be screened for HBV and HCV. Screening for HCV with antibodies is routine among SOT candidates. A reactive or indeterminate/equivocal antibody test should be followed by HCV RNA testing. Transplant candidates on dialysis are a challenging population for HCV screening as patients may have false negatives due to their immunosuppressed state. False positives in this population are also a problem. Due to these complexities, some transplant centers opt for universal HCV RNA screening for patients on dialysis.
HCV infection in potential recipients is not considered a contraindication to transplantation, unless the patient has cirrhosis. The rationale for screening patients is that HCV infection may be successfully treated either prior to or after transplantation. In addition, knowing HCV status enables allocation of organs from HCV-positive donors to HCV-positive recipients.
Screening tests for HBV include hepatitis B surface antigen, surface antibody, and core antibody. These assays help stratify the risk of reactivation and allocate organs from donors who have evidence of prior HBV infection.
Laboratories routinely perform serological testing to determine prior infection with CMV (CMV IgG) and EBV (EBNA IgG, EBV VCA IgM and IgG, and EA IgG). These assays help stratify the risk of CMV disease and EBV-associated post-transplant lymphoproliferative disorder.
Serological testing for evidence of prior infection or immunity (with IgG) for varicella, measles, mumps, and rubella also is recommended. These screening results, in combination with vaccination records, aid in the decision to update the vaccination status of recipients prior to transplantation.
Screening donors, both living and deceased, is pivotal in avoiding unintended donor-derived infection acquired through transplantation. The differences in screening of the living donor and the deceased donor are largely based on the different time constraints during which the evaluation must take place. Recent comprehensive reviews have explored this subject (1, 13, 14).
Infections remain a major complication of SOT, so screening both donors and recipients is essential to positive clinical outcomes in these life-changing procedures. Pre-transplant evaluations involve an in-depth process that includes a detailed history and physical exam along with accurate diagnostic screening. For this reason, clinical laboratorians play a key role in the success of transplant programs.
Ricardo M. La Hoz, MD, FACP, is an assistant professor of internal medicine in the division of infectious diseases at the University of Texas Southwestern Medical Center in Dallas, Texas. +Email: Ricardo.LaHoz@UTSouthwestern.edu
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