INTRODUCTION Among patients who have undergone liver transplantation, liver enzymes (aminotransferases, bilirubin, and alkaline phosphatase) provide an important indicator of graft function, anatomic and biliary complications, and immunologic response. The pattern of elevation in conjunction with the timing and clinical context can offer insight into the mechanism of injury.1 Frequently, an elevation in liver enzymes is the first indicator of graft pathology, providing an opportunity to intervene clinically to preserve allograft function. PATTERNS OF LIVER ENZYME ELEVATION Across the lifetime of a post-transplant patient, complications directly affecting the liver allograft typically fall into several broad categories: early operative injury, vascular and biliary complications, immune-mediated injury, drugs, infectious complications, and recurrence of primary liver disease (Tables 1–3). Cholestatic and hepatocellular liver enzyme patterns can be suggestive of specific diagnoses, but, in many cases, post-transplant complications demonstrate a relatively nonspecific mixed liver enzyme pattern.2 Management of patients, therefore, often relies on a careful clinical history and a nuanced understanding of symptoms, culture data, serologic tests, and imaging. Ultrasound with Doppler, cross-sectional imaging, endoscopic retrograde cholangiopancreatography, and liver biopsy are among the most frequent tools used in the evaluation of post-transplant elevated liver enzymes. In addition, the rate of rise in liver enzymes often portends more potential harm to the liver allograft and can require rapid diagnostic evaluation to guide appropriate intervention. Dynamic physiological and immunologic changes, particularly early post-transplant, can often lead to co-occurring processes that can complicate the identification of otherwise characteristic liver enzyme pattern presentations.3 Therefore, while the pattern of liver enzyme elevation and rate of rise is important, these changes must also be interpreted in a clinical context to allow prompt diagnosis. TABLE 1 - Perioperative, vascular, and biliary causes of abnormal liver enzymes after LT Category Diagnosis Definition Timing Pattern Risk factors Evaluation Treatment Perioperative complications Ischemia reperfusion injury/preservation injury Hepatocellular graft damage that results from perioperative cold and warm ischemic time as well as reperfusion of the donor liver Post-transplant rise in liver enzymes; typically peaks in the first 2–7 d and spontaneously regresses AST/ALT 5-20X ULN Delayed rise in total bilirubin can occur Typically seen to varying degrees in all patients after transplant None Observation PNF Widespread hepatocellular necrosis and hemorrhage, evolving into post-transplant liver failure. PNF Defined as: 1.AST > 3000 2.INR > 2.5 3.Within 7 d of transplantation Within first 7 d AST >3000 IU/ML and INR > 2.5 within 7 d post-LT DCD allograft >12 h cold ischemic time >90 min warm ischemic time Imaging to rule out vascular obstruction Retransplant (listed status 1A) Fluid collection, abscess Perioperative fluid collections can become infected in the absence of source control First 1–2 wk post-transplant Mild liver enzyme pattern, cholestasis of sepsis Hematoma or biloma formation Reoperation/take-back Cross-sectional imaging, labs Drainage of collection Vascular complications Early HAT Acute thrombosis of the hepatic artery associated with a wide range of clinical manifestations including fever, sudden liver enzyme rise, fulminant hepatic failure Median 7 d, within first 2–3 wk after transplantation Acute ischemic injury with sharp rise in ALT and AST typically >10× ULN Arterial reconstruction Delayed reperfusion Multiple anastomoses Urgent duplex ultrasound or CT angiogram Endovascular intervention, occasionally retransplant Late HAT Narrowing of the transverse diameter of HA by 50%, with resistive index <0.5, peak systolic velocity >400 Can be early or late post-transplant, often insidious. Median 3 mo post-transplant Cholestatic pattern due to diffuse biliary stricturing with elevation in AP +/− TB Intraoperative factors (clamp injury, intimal dissection) and donor anatomy Doppler ultrasound Endovascular intervention Surgical revision Biliary complications Biliary stricture (anastomotic and diffuse type) Focal stricturing at the biliary anastomoses or the diffuse stricturing of large intrahepatic or extrahepatic bile ducts of the donor liver. Within first 2–3 wk after transplantation AP 2–5× ULN with elevated TB (predominantly direct) Surgical technique, ischemic time, hypotension ERCP Endoscopic stenting or balloon dilation. May require revision to Roux-en-y. Bile leak Biliary anastomotic leak, typically producing a fluid collection. Reduced drainage from biliary drain. Most commonly within 30 d Tbili elevation No specific risk factors CT to evaluate collection ERCP vs IR guided stenting and drainage Acute cholangitis Rising bilirubin related to obstruction and infection in the bile ducts, often related to stricturing Any time, often within 1–2 wk Tbili and alk phos predominant rise, leukocytosis Duct anastomosis without drain LDLT MRCP ERCP or PTBD tube Drug related complications DILI Multiple postoperative drug exposures including antibiotics, steroids, TPN, and IS (cyclosporine and azathioprine) Following drug administration cholestatic injury common, less common hepatocellular injury but state dependent on agent Drug exposures Liver biopsy, but nonspecific Removal of causative drug Abbreviations: ALT, alanine aminotransferase; AP, alkaline phosphatase; AST, aspartate aminotransferase; ERCP, endoscopic retrograde cholangiopancreatography; HA, hepatic artery; HAT, hepatic artery thrombosis; INR, international normalized ratio; IR, interventional radiology; IS, immunosuppression; LDLT, living donor liver transplantation; LT, liver transplantation; MRCP, magnetic resonance cholangiopancreatography; PNF, primary nonfunction; PTBD, percutaneous transhepatic biliary drainage; TB, total bilirubin; TPN, total parenteral nutrition; ULN, upper limit of normal. TABLE 2 - Immune-mediated causes of abnormal liver enzymes after LT Category Diagnosis Definition Timing Pattern Risk factors Evaluation Treatment T-cell mediated complications Acute T-cell mediated rejection T-cell mediated inflammatory infiltrate causing damage to the bile ducts, portal tract, and endothelium (endotheliitis), not associated with low IS levels Later episodes typically occur in the setting of inadequate IS Median 7–10 d post-transplant, most commonly within the first 90 d AST/ALT typically 5–15× ULN Mild elevation in AP 2–5× ULN Elevation in bilirubin only seen in severe ACR Immune-mediated liver disease Inadequate IS Infection, viremia Liver biopsy Short term increase in IS +/− steroid burst depending on severity Chronic T-cell mediated rejection Destruction and loss of bile ducts (vanishing bile duct) with obliteration of the small hepatic arteries. Often preceded by ACR which does not respond to increased IS. 6 wk–6 m after LT AP elevation to 2–5× ULN +/− elevation in TB Inadequate IS levels or compliance Liver biopsy Increased IS Antibody-mediated complications Acute AMR Rapid graft failure caused by preformed circulating antibodies directed against donor antigens. Causes endothelial damage and necrosis. Rare in liver transplant. Typically only if ABO mismatch. 4 pathologic criteria for diagnosis: microvascular inflammation and portal edema, elevated DSA, C4d deposition, exclusion of other liver diseases Usually within first 3–4 wk post-transplant AST/ALT typically 5–15× ULN Mild elevation in AP 2–5× ULN Elevation in bilirubin only seen in severe ACR ABO mismatch Serum evaluation for DSA Liver biopsy with staining for C4d IVIG treatment Rituximab Plasmapheresis Bortezumib Chronic AMR Can be associated with graft injury and/or advanced fibrosis, abnormal liver enzymes after weaning IS Pathologic features: mild portal inflammation, mild interface hepatitis, dense portal fibrosis. Positive DSA within 3 mo of biopsy. Focal C4d positivity ( >10% portal tracts). Exclusion of other causes Any time after the first month after transplantation Similar to early AMR but typically more subtle ABO mismatch Serum evaluation for DSA Liver biopsy with staining for C4d Retransplant Other rejection Plasma cell rich rejection A manifestation of allograft rejection with positive C4d staining of portal capillaries. Rapid progression of fibrosis resistant to IS. Any time after transplant AP elevation Unknown C4d staining of portal capillaries Retransplant Abbreviations: ACR, acute cellular rejection; ALT, alanine aminotransferase; AMR, antibody-mediated rejection; AP, alkaline phosphatase; AST, aspartate aminotransferase; DSA, donor-specific antigen; IS, immunosuppression; IVIG, intravenous immunoglobulin; LT, liver transplantation; TB, total bilirubin; ULN, upper limit of normal. TABLE 3 - Infectious causes of abnormal liver enzymes after LT Category Diagnosis Findings Timing Pattern Risk factors Evaluation Treatment Viral infections CMV infection Fever, leukopenia, elevated liver enzymes, diarrhea Most common in the first 1–6 mo after transplantation AST/ALT 5–15× ULN Most common in CMV D + /R− combinations Serum CMV PCR Colonoscopy with biopsies PPX: valganciclovir Treatment: valganciclovir HBV (donor-derived or reactivation) Mild elevation in liver enzymes, fever, or asymptomatic Transmission within 3–5 d AST/ALT 1–3× ULN Known infections in donor Day 3 PCR, Day 7 PCR, weekly thereafter Entecavir or Tenofovir for suppression Donor-derived HCV Mild elevation in liver enzymes, fever, or asymptomatic Transmission most commonly 3–5 d, can occur in 3–6 mo AST/ALT 1–3× ULN Known infections in donor Day 3 PCR, Day 7 PCR, weekly thereafter Early initiation of DAA HSV Fever, fatigue, severely elevated liver enzymes, leukopenia; note that rash is not required Most common in the first 3 mo after transplantation AST/ALT 5–15× ULN Most common in new HSV acquisition HSV serum PCR Valacyclovir SARS-CoV-2 Fever, liver injury, upper respiratory symptoms Anytime AST/ALT 1–3× ULN Metabolic syndrome SARS-CoV-2 PCR Monoclonal antibody Remdesivir, prednisone Bacterial infections CDI Diarrhea, fulminant colitis Higher risk in the first 3 mo Elevation in liver enzymes only if severe colitis, dehydration present Exposure to multiple antimicrobial agents, prior CDI C.diff EIA and toxin Fidaxomicin PO Vancomycin Metronidazole Liver abscess Abdominal pain, fevers, intrahepatic collections Anytime Alk phos/ Tbili 5–10× ULN Ischemic injury, bile leak CT or MRI IR drainage and IV antibiotics Ascending cholangitis Abdominal pain, fevers, biliary stricture and biliary dilatation Anytime Alk phos/ Tbili 5–10× ULN Anastomotic stricture MRCP or ERCP ERCP vs percutaneous drainage Cholestasis of sepsis Hypotension, tachycardia, localizing infectious symptoms Anytime Alk phos/ Tbili 5-10x ULN Sepsis of any kind (urinary, SSTI, abscess, PNA) None Treatment of underlying source infection Fungal infections Invasive candidal infections Candidemia (sepsis) line infection, peritonitis, endophthalmitis First 3 mo Typically elevations are associated with cholestasis of sepsis CMV infection Line and blood cultures, fluid culture Fluconazole (mild-moderate infection, CNI interaction) Echinocandin (moderate-severe infection) Aspergillus CT chest: nodular opacity with halo sign, 50% of the time aspergillus becomes invasive First 30 d Typically elevations are associated with cholestasis of sepsis Pretransplant colonization Sputum culture Chest imaging B-D-glucan Galactomannan Voriconazole Endemic Mycoses (Histoplasmosis, Coccidiomycosis, Blastomycosis) Pneumonia, 30% with disseminated disease: hepatosplenomegaly, GI involvement, sepsis First 3 mo Elevation of Tbili/Alk phos related to hepatic parenchymal invasion or cholestasis of sepsis Pretransplant exposure through endemic location or donor-derived infection Serologic testing Cross-sectional imaging Itraconazole Amphotericin B Abbreviations: ALT, alanine aminotransferase; AST, aspartate aminotransferase; CDI, Clostridium Difficile infection; CMV, cytomegalovirus; CNI, calcineurin inhibitor; DAA, direct-acting antiviral; EIA, enzyme immunoassay; ERCP, endoscopic retrograde cholangiopancreatography; GI, gastrointestinal; HSV, Herpes simplex virus; IR, interventional radiology; LT, liver transplantation; MRCP, magnetic resonance cholangiopancreatography; PO, per os; PPX, prophylaxis; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2; ULN, upper limit of normal. TIMING OF LIVER ENZYME ELEVATION The length of time since transplant is among the most distinguishing features of post-transplant elevated liver enzymes (Figure 1). Although several complications, such as DILI and infection, can occur at any time, a majority of the presentations occur within a very specific window post-transplant.4 Findings can be categorized as immediate (first week), early (first month), intermediate (1–12 mo), and late complications (>1 y).FIGURE 1: Causes of liver enzyme elevations over the post-transplant period. The causes of liver enzyme elevations post-transplant are shown relative to the time from transplant, during which these causes are most likely to occur. Abbreviations: CMV, cytomegalovirus; HAT, hepatic artery thrombosis; HSV, Herpes simplex virus; IS, immunosuppression; LT, liver transplantation.IMMEDIATE PERIOD (DAYS 0–7) During the first week after the transplant, almost all patients experience a transient elevation in liver enzymes. This is attributed to a combination of operative “preservation injury” and ischemic reperfusion injury – injury to the donor liver resulting from cold and warm ischemic time and vascular reperfusion.5 Classically, preservation injury is an AST/ALT predominant pattern, which rises steadily over ~7 days to 5–20x the ULN, often with a delayed elevation in bilirubin.5 The challenge in the immediate postoperative period is to distinguish preservation injury from other more acute complications requiring urgent intervention: primary nonfunction, hepatic artery thrombosis, biliary complications, and early acute cellular rejection (ACR) (Tables 1, 2). Early Doppler ultrasound to evaluate vascular patency is appropriate in the first 24 hours after transplant and after any acute change in graft function.6 Improving the international normalized ratio distinguishes rising liver enzymes of preservation injury from graft-threatening complications, including primary nonfunction and hepatic artery thrombosis.7 Other immediate complications include biliary leaks, which occur in 2%–25% of patients after transplant.6 An early isolated bilirubin elevation, often in combination with abdominal pain, leukocytosis, and fevers should prompt cross-sectional imaging to look for fluid collections requiring endoscopic or percutaneous drainage. Hemodynamic complications, including bleeding and volume overload, can also lead to mixed elevations in liver enzymes driven by perioperative hypotension, ischemia, and hepatic congestion. In addition, resorption of hematomas and transfusion of red blood cells can result in transient rises in indirect bilirubin that does not reflect allograft dysfunction. Because of the complex physiological changes that occur in the first-week post-transplant, empiric treatment of complications is not uncommon (eg, escalation of glucocorticoids for empiric treatment of ACR). EARLY PERIOD (<1 MONTH) ACR most frequently occurs in the first 30 days after transplant with a mixed elevation in liver enzymes. Liver enzymes that do not decline after the first postoperative week or rise again after a period of improvement should prompt evaluation for ACR with liver biopsy. Attempts to identify serum biomarkers for ACR are in the early stages, and liver biopsy remains the gold standard. A 2022 study by Levitsky et al8 demonstrated the validity of a 59-gene biomarker, which succeeds in distinguishing transplant recipients with acute rejection. Since the introduction of tacrolimus-based regimens, published estimates of the incidence of ACR range from 15% to 45%.4,9,10 A 2016 prospective study by Shindoh et al11 observed a median time to ACR of 17 days (range 5–83 d) after transplantation. Early ACR, most commonly defined as occurring within the first 6 weeks post-transplant, occurs in an estimated 30%–35% of patients.12 Early ACR is treated effectively with a burst of steroids or escalation of immunosuppression (IS).13,14 While initial studies demonstrated that ACR had no impact on patient and graft survival, more recent data refute that assertion. A 2019 study by Jadlowiec et al, however, demonstrated that 17% of patients experienced late ACR episodes after the first 6 weeks post-transplant.12 Episodes of late ACR were associated with a higher rate of chronic rejection and graft failure.15,16 In addition, Levitsky et al10 demonstrated that, in 2 large national databases, biopsy-proven ACR was associated with an increased risk of both graft loss and death in the 12 months after diagnosis. The incidence of ACR is substantially higher in individuals with autoimmune etiology of liver disease, and it is important to recognize that early ACR is not prevented by adequate serum IS levels.4 In patients refractory to treatment, antibody-mediated rejection and plasma-cell–rich rejection should be considered with additional diagnostic testing to include donor-specific antigen testing and C4d liver tissue staining (in addition to routine hematoxylin and eosin liver tissue staining). Because IS takes some time to reach maximum efficacy, the first postoperative month is the highest risk for ACR and lowest risk for opportunistic infections. During the early postoperative period, a majority of infections are bacterial complications of the operation and hospitalization itself: wound infections, acute cholangitis, abdominal fluid collections, urinary infections, and Clostridium difficile colitis.2 Other early complications include anastomotic biliary stricture and obstructive jaundice, with or without acute cholangitis.6 If there is a high degree of suspicion for stricture, cross-sectional imaging with CT or MRI is typically followed by endoscopic retrograde cholangiopancreatography for endoscopic stenting. Some patients may ultimately require revision to roux-en-y anatomy to achieve long-term resolution of obstruction. INTERMEDIATE PERIOD (1–12 MONTHS) Beginning at ~2–3 months post-transplant, antimicrobial prophylaxis and immunosuppressive regimens begin to be weaned. This phase of post-transplant management demands careful attention to the balance between the simultaneous risks of infection and rejection, requiring an individualized approach guided by patient response. Stepwise de-escalation of IS should be combined with careful monitoring of liver enzymes. During this phase, late-onset ACR often occurs in patients who are young, female, or have an autoimmune history.4 In contrast to early ACR, late rejection is associated with immunosuppressive levels and should be suspected if even very mild elevations in liver enzymes develop after a change in immunosuppressive medication dosing. At the same time, the risk of developing an opportunistic infection increases with the tapering of prophylaxis. Viral infections, particularly cytomegalovirus, carry an important risk of activating the host's innate immune response and triggering an episode of ACR. Elevations in liver enzymes during this phase should, therefore, also prompt consideration of cytomegalovirus and other viral infections depending on the clinical context. Candidiasis, aspergillosis, P. jirovecii pneumonia, and endemic mycoses are other important causes of post-transplant opportunistic infection but present typically with pulmonary or tissue-invasive disease and so are less likely to increase a patient’s liver enzymes in the absence of disseminated disease.2 Finding a careful balance between immune-mediated and infectious complications is critical to long-term graft survival Early disease recurrence, particularly NASH, is also common; liver biopsy may be required to distinguish this from rejection. Biliary complications frequently arise during this time period and include both anastomotic and nonanastomotic strictures. LATE PERIOD (>12 MONTHS) After ~12 months post-transplant, elevations in liver enzymes become less common as the patient’s immune response and immunosuppressive dosing stabilize. In patients with chronically low IS, ductopenic rejection causes rising bilirubin with progressive bile duct loss on liver biopsy, often irreversible. Chronic rejection involves the loss of at least 50% of portal tracts and classic foam cell obliterative arteriopathy.15 This type of immune-mediated duct loss is irreversible and occurs in an estimated 2%–5% of transplant recipients.17 Given advances in immunosuppressants, chronic rejection is frequently associated with poor medication adherence and should, therefore, prompt evaluation of medication-related issues, such as access and affordability.15 In addition to rejection, post-transplant alcohol use and weight gain represent a significant risk to allograft function. Studies suggest that about 10% of patients use alcohol heavily after transplant, regardless of whether the patient’s original liver disease was alcohol-associated.18 Elevated liver enzymes of unclear etiology post-transplant should, therefore, prompt additional substance use history and alcohol biomarker testing. It is important to educate patients in advance that both alcohol use and recurrent NASH can cause the rapid development of allograft cirrhosis.19 Autoimmune liver diseases may also reoccur or develop de-novo in a post-transplant patient. Autoantibody testing is not reliable due to IS, so identifying autoimmune liver disease requires a high index of clinical suspicion and judicious use of liver biopsy for confirmation.20 PSC, in particular, is associated with an increased risk of severe recurrence; an estimated 30% of those with recurrent PSC will ultimately require re-transplantation.21 As patients progress into the second and third decades after transplant, the risks of graft dysfunction decrease, allowing for lower levels of IS and reduced frequency of lab monitoring over time. CONCLUSION Liver enzymes provide the most reliable noninvasive method for evaluating clinical status after liver transplantation. Regular monitoring is required throughout the lifetime of a liver transplant recipient. It is critical that clinicians managing post-transplant patients recognize the most common presentations of liver enzyme elevations and the appropriate first steps in evaluation, diagnosis, and treatment to preserve long-term allograft function.