Oral glucose tolerance test catches hidden diabetes in kidney transplant candidates

Roughly one in twelve Norwegian kidney transplant candidates who believed they were diabetes-free actually had blood sugar values falling within the diabetes range when screened before being placed on the waiting list, and the overwhelming majority of those cases would have slipped through undetected had doctors relied only on the two most common screening tests. That is the central message of a nationwide analysis published in Frontiers in Nephrology, which argues that the oral glucose tolerance test, or OGTT, deserves its place as the gold standard for glycaemic assessment before transplantation.

The study, conducted by a team of clinicians and researchers at Oslo University Hospital, the University of Oslo and the Norwegian Renal Registry, was a retrospective observational investigation drawing on the complete national cohort of kidney transplant recipients in Norway between 2007 and 2022. The authors identified 3,290 people who received a kidney transplant during that fifteen-year window and then narrowed the pool to 2,052 eligible participants by excluding those with pre-existing diabetes, anyone listed for re-transplantation or combined organ transplantation, and patients lacking available HbA1c or OGTT results. All patients had provided signed informed consent for this type of analysis, and the Regional Committee for Medical and Health Research Ethics South-East approved the study under reference 407293.

Because Norway runs a systematic, standardized pretransplant workup in which every candidate without known diabetes routinely undergoes both HbA1c testing and an OGTT, the researchers had an unusually complete dataset to examine how different diagnostic thresholds behave in the same patients. HbA1c analyses have been standardized across Norwegian hospitals since 2007 using high-performance liquid chromatography or immunochemical methods certified by the IFCC in accredited laboratories. The OGTT itself followed a strict protocol: performed in the morning after at least ten hours of fasting, with seventy-five grams of glucose dissolved in 200 millilitres of water administered while the patient sat quietly, having avoided smoking that day, maintained a weight-stabilizing diet for three days beforehand and abstained from alcohol. Plasma glucose was measured immediately before the glucose load and again thirty and 120 minutes afterwards.

The diagnostic yardstick came from the American Diabetes Association, which defines diabetes-range glycaemic levels as an HbA1c of at least 48 mmol/mol, a fasting plasma glucose of at least 7.0 mmol/L, or a two-hour plasma glucose of at least 11.1 mmol/L during a 75-gram OGTT. Prediabetes-range levels were defined as HbA1c between 39 and 47 mmol/mol, fasting plasma glucose between 5.6 and 6.9 mmol/L, or two-hour plasma glucose between 7.8 and 11.0 mmol/L. Where patients had repeated tests, the analysis used only the OGTT performed closest to the time of waitlisting.

The headline result was striking. Among the 2,052 recipients without known diabetes, 165 patients, or 8 percent, had at least one glycaemic test result within the diabetes range detected during the pretransplant workup. The cohort was 66 percent male, with a mean age at transplantation of 54 years ranging from 18 to 83, and a mean body mass index of 25.8 kg/m2. Thirty percent were transplanted pre-emptively, 47 percent had been on haemodialysis and 23 percent on peritoneal dialysis. The mean interval between the glycaemic assessment and transplantation was 1.62 years, with a range of zero to eight years.

The breakdown of which test caught those 165 cases is where the clinical argument becomes compelling. Only 48 patients fulfilled the HbA1c criterion and 23 fulfilled the fasting plasma glucose criterion, while 122 fulfilled the two-hour plasma glucose criterion, with 23 patients meeting more than one threshold. In practical terms, 74 percent of the diabetes-range findings were identified through the two-hour value alone. When the authors recalculated the figures without the two-hour glucose results, only 65 patients, or 39 percent of the total, would have been flagged. Skipping the OGTT and relying on HbA1c and fasting glucose together, in other words, missed roughly six out of every ten cases in this cohort. Most of those patients, the authors write, would likely never have proceeded to further diagnostics or follow-up.

Diabetes matters enormously in the transplant setting. The presence of diabetes mellitus adversely affects outcomes after kidney transplantation, and both prediabetes and diabetes are associated with increased mortality and cardiovascular risk after the operation. Diabetes that first appears after transplantation, known as post-transplant diabetes mellitus, is a recognized complication, and the 2022 PTDM Consensus Meeting recommended using pretransplant test results, including OGTT data, in a continuous risk assessment for this outcome. The authors also note that some transplant centres consider less diabetogenic immunosuppressive regimens for patients judged to be at elevated risk, an option that becomes available only if the risk is actually identified beforehand.

Guidelines already point in the direction the Norwegian data support. The KDIGO guidelines regard the OGTT as the gold standard for pretransplant glycaemic assessment, and the American Diabetes Association criteria allow diagnosis on the basis of HbA1c, fasting plasma glucose or the two-hour OGTT value, with confirmatory testing required in asymptomatic patients. Yet practice varies widely. A European survey by the ERA DESCARTES Working Group found that the KDIGO recommendation has not been universally implemented, and some transplant centres continue to prefer HbA1c and fasting glucose alone. There is even precedent from another field: research on gestational diabetes screening has examined how women tolerate different testing approaches, underscoring that the choice of screening method is never purely a technical matter.

Why the two-hour value is so much more sensitive in this population remains a question the study cannot fully answer, but the authors point to known discrepancies between HbA1c and OGTT results in the general population and among kidney-transplanted patients, as well as the established limitations of HbA1c in reflecting glycaemia accurately in chronic kidney disease. Among the 1,887 patients who did not reach diabetes-range values, more than half, 976 patients or 52 percent, fell within the prediabetes range by one or more ADA criteria, with 514 meeting the HbA1c criterion, 417 the fasting glucose criterion and 449 the two-hour criterion. The findings echo smaller studies from Austria and Spain, which detected diabetes-range screening results in 3 and 6 percent of waitlisted candidates respectively.

The authors are candid about what their study does and does not show. They lacked information on whether positive screening results actually triggered confirmatory re-testing, diabetes treatment or preventive measures, and they recommend such follow-up be implemented, noting that hyperglycaemia identified before transplantation can inform tailored lifestyle or pharmacological interventions, and that weight loss in cases of overweight or obesity can prevent diabetes or even push it into remission when caught early. They also flag methodological limits: each patient’s HbA1c and OGTT were measured only once, whereas confirmatory testing is advocated before diagnosing diabetes in asymptomatic people; the OGTT is well known to have limited reproducibility; the cohort was predominantly Caucasian and consisted of patients who ultimately received transplants, which limits generalizability to all transplant candidates and other ethnic groups; and data on anaemia treatment, blood transfusions, erythropoiesis-stimulating agents and iron supplementation were unavailable. Excluded patients were on average five years younger than those included, though baseline BMI was comparable between the groups.

As an opinion piece backed by national registry data rather than a randomized trial, the article stops short of prescribing practice change outright, but its conclusion is pointed. In this complete national cohort, 8 percent of candidates carried previously unrecognized diabetes-range glycaemia, and most of those cases surfaced only through the two-hour plasma glucose measurement. The authors suggest that centres relying on HbA1c and fasting plasma glucose alone may be missing patients who warrant further diagnostics and follow-up, and that the standardized OGTT embedded in the Norwegian workup offers a model worth examining wherever transplant programmes still screen with less complete tools.

Subject of Research: Glycaemic screening with oral glucose tolerance testing before kidney transplantation in Norway

Article Title: Glycaemic screening before kidney transplantation should include an oral glucose tolerance test

Article References: Nordheim, E., Jenssen, T. G., Lindahl, J. P., Birkeland, K. I., Midtvedt, K., Heldal, K., Hagness, M., Hovd, M., & Åsberg, A. (2026). Glycaemic screening before kidney transplantation should include an oral glucose tolerance test. Frontiers in Nephrology, 6, Article 1849545. https://doi.org/10.3389/fneph.2026.1849545

Image Credits: AI Generated

DOI: 10.3389/fneph.2026.1849545

Keywords: kidney transplantation, oral glucose tolerance test, HbA1c, diabetes screening, prediabetes, post-transplant diabetes mellitus, KDIGO guidelines, Norway, chronic kidney disease, transplant waitlist, fasting plasma glucose, American Diabetes Association