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Cardiorenal Nexus in ATTR-Amyloidosis: Beyond eGFR—Leveraging Albuminuria for Enhanced Risk Stratification

This longitudinal study demonstrates that urine albumin-to-creatinine ratio (UACR) serves as a sensitive early marker for systemic disease progression in transthyretin amyloid cardiomyopathy, offering enhanced risk stratification beyond traditional eGFR-based models by predicting advancement to higher National Amyloidosis Centre stages even when renal function appears preserved.

Original authors: Mahshid Eslami, Mariusz Tadeusz Grzeda, Antonio Kösters, Nikita Ermolaev, Stefanie Pahr, Bernhard Gregshammer, Lisa Seilinger, Leonhard Lichtblau, Begüm Öztürk, Michael Wolzt, Jutta Bergler-Klein, Joh

Published 2026-09-24

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Mahshid Eslami, Mariusz Tadeusz Grzeda, Antonio Kösters, Nikita Ermolaev, Stefanie Pahr, Bernhard Gregshammer, Lisa Seilinger, Leonhard Lichtblau, Begüm Öztürk, Michael Wolzt, Jutta Bergler-Klein, Johannes Kastner, Helmuth Haslacher, Roza Badr Eslam

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The heart and kidneys are partners in a delicate, continuous exchange. When the heart struggles to pump effectively, pressure builds up in the veins, sending a ripple effect back to the kidneys, which rely on steady pressure to filter waste from the blood. Conversely, when kidneys falter, fluid and toxins accumulate, forcing the heart to work even harder. This partnership is so intertwined that doctors often look at how well the kidneys are filtering blood to gauge the severity of heart disease. For years, the standard way to measure this kidney function has been a calculation based on a blood test called creatinine, known as the estimated glomerular filtration rate, or eGFR. This number tells doctors how much blood the kidneys clean every minute. However, this measurement has a blind spot: it often remains normal until the kidneys have already suffered significant, irreversible damage. Just as a car's engine might run smoothly even while internal parts are wearing down, the kidneys can filter blood efficiently while their delicate internal structures are quietly being injured.

In the specific case of a condition called transthyretin amyloid cardiomyopathy, this blind spot becomes a critical diagnostic gap. This disease occurs when a misshapen protein called amyloid builds up in the heart muscle, making it stiff and unable to pump properly. As the heart stiffens, it creates a backlog of pressure that travels down to the kidneys. For a long time, medical guidelines focused almost entirely on the filtration rate to stage the severity of the disease and predict a patient's future. But researchers at the Medical University of Vienna suspected that this approach was waiting too long to sound the alarm. They wondered if a different sign of kidney stress, one that appears before the filtration rate drops, could reveal the true burden of the disease much earlier.

To investigate this, the team looked at 124 patients diagnosed with this heart condition. Instead of just watching the filtration rate, they examined a different marker in the urine: the amount of albumin, a type of protein, leaking out. In a healthy kidney, the filter is tight enough to keep protein in the blood. When the kidney's structure is damaged or under pressure, this filter becomes leaky, and protein spills into the urine. The researchers measured this leakage using a ratio called the urine albumin-to-creatinine ratio, or UACR. They then tracked these patients over time to see if the amount of protein in their urine at the start could predict how sick they would become, even if their blood filtration numbers looked normal.

The study found that the traditional view of kidney health in this disease was incomplete. While the filtration rate remained a strong indicator of how sick a patient was overall, the amount of protein in the urine told a different, earlier story. The researchers discovered that high levels of protein leakage did not immediately cause the filtration rate to drop. Instead, the protein leakage acted as a sensitive early warning system. It signaled that the heart was already putting significant stress on the kidneys, causing structural damage that had not yet slowed down the filtering process.

When the team analyzed the data, they saw a clear pattern. Patients who had normal filtration rates but higher levels of protein in their urine were much more likely to progress to more severe stages of the disease compared to those with low levels of protein. For instance, in a patient with a perfectly normal filtration rate, increasing the protein leakage from a low level to a high level changed the odds of remaining in the mildest stage of the disease from about 77 percent down to 41 percent. At the same time, the likelihood of moving to a more advanced stage of the disease more than doubled. This shift happened even though the blood test for filtration function had not changed at all.

The findings suggest that the protein leakage is not just a side effect but a direct result of the heart's pressure pushing back against the kidneys. In this specific disease, the amyloid protein builds up in the heart first, raising the pressure in the veins. This pressure travels to the kidneys and forces the tiny filters to leak protein long before the filters themselves break down and stop working. The study indicates that relying solely on the filtration rate is like checking a car's speedometer while ignoring the engine temperature gauge; the car might still be moving, but the engine is already overheating.

This research does not claim that the protein measurement replaces the filtration test, but rather that it must be used alongside it. By adding the protein measurement to the standard assessment, doctors could potentially identify patients who are at higher risk of worsening disease much earlier than before. This earlier detection could open a window of time where treatments might be more effective, allowing doctors to intervene before the damage becomes permanent. The study concludes that for patients with this type of heart disease, looking at the protein in the urine provides a clearer, earlier picture of the struggle between the heart and kidneys, offering a chance to catch the disease's progression before it is too late.

Technical Summary: Cardiorenal Nexus in ATTR-Amyloidosis

Problem Statement
Transthyretin amyloid cardiomyopathy (ATTR-CM) is a progressive systemic disease characterized by amyloid protein accumulation in the heart, leading to heart failure and renal congestion. Current risk stratification relies heavily on the National Amyloidosis Centre (NAC) staging system, which incorporates the estimated glomerular filtration rate (eGFR) as a marker of renal function. However, eGFR is a late indicator of organ damage, typically declining only after irreversible nephron loss. Consequently, patients with early-stage ATTR-CM may be misclassified as "low risk" if their filtration rate remains preserved, despite the presence of significant venous congestion and subclinical glomerular injury. The study addresses the diagnostic gap in identifying early systemic disease burden before measurable functional decline occurs.

Methodology
This retrospective longitudinal study analyzed 124 patients with biopsy- or scintigraphy-confirmed ATTR-CM enrolled at the Medical University of Vienna. The research applied the Kidney Disease: Improving Global Outcomes (KDIGO) 2024 Risk Matrix framework, which integrates filtration function (eGFR) with structural kidney damage markers (urine albumin-to-creatinine ratio, or UACR).

The analysis proceeded in two stages:

Descriptive Analysis: Characterization of the cohort's renal risk categories and exploration of associations between renal markers (UACR, eGFR) and clinical staging (NAC stages I–III).
Generalised Structural Equation Modeling (GSEM): A statistical model was constructed to evaluate the hypothesized trajectory where baseline structural injury (log-transformed UACR) predicts subsequent functional decline (follow-up log-transformed eGFR) and overall disease severity (follow-up NAC stage). The model treated NAC stage as an ordinal outcome using a probit link function. It estimated both direct effects of baseline UACR on follow-up NAC stage and indirect effects mediated through changes in follow-up eGFR.

Key Results

Cohort Characteristics: The median age was 80.2 years (88.7% male). The median baseline eGFR was 61.2 mL/min/1.73 m², while UACR values showed wide variability, with many patients exhibiting albuminuria despite preserved filtration.
GSEM Findings:
- Direct vs. Indirect Pathways: Baseline UACR was not directly associated with follow-up eGFR ( $\beta = -0.0013, p = 0.971$ ), indicating that albuminuria does not immediately cause measurable functional loss in this cohort. While baseline UACR showed a positive association with subsequent NAC stage ( $\beta = 0.365, p = 0.068$ ), the paper notes that this association did not reach conventional statistical significance, though the direction and magnitude were consistent with the proposed role of UACR as an early marker.
- Follow-up eGFR: Follow-up eGFR remained a strong inverse predictor of NAC stage ( $\beta = -5.53, p < 0.001$ ), confirming the established link between renal function and systemic disease burden.
Predicted Probabilities: When holding eGFR constant at 60 mL/min/1.73 m², increasing UACR from 30 mg/g to 400 mg/g was associated with a shift in the probability of disease staging:
- The probability of remaining in NAC Stage I decreased from 76.6% to 41.3%.
- The probability of progressing to NAC Stage II increased from 23.2% to 56.7%.
- The probability of NAC Stage III, while low, increased 20-fold across the same range.

Key Contributions

Early Detection Marker: The study identifies UACR as a sensitive early indicator of systemic disease in ATTR-CM, capable of detecting subclinical cardiac and renal involvement before eGFR declines.
Mechanistic Insight: The GSEM results suggest that in ATTR-CM, renal stress is driven by systemic venous congestion (elevated right-sided heart pressure transmitting backpressure to renal veins) rather than direct amyloid deposition in the kidney. This results in "subclinical cardiorenal decoupling," where protein leakage (albuminuria) occurs while filtration remains stable.
Risk Stratification Refinement: The research demonstrates that relying solely on eGFR can misclassify patients. Incorporating UACR alongside eGFR provides a more granular risk gradient, distinguishing patients with preserved filtration but high structural damage who are at greater risk of progression.

Significance and Claims
The paper suggests that UACR serves as a potential prognostic marker that predicts future progression to higher NAC stages, even when renal filtration is preserved. By translating statistical associations into clinically interpretable probabilities, the study argues that routine UACR assessment should be integrated with eGFR to reduce diagnostic blind spots.

The authors posit that this approach facilitates earlier detection of disease burden, potentially opening a therapeutic window for the initiation of organ-protective therapies (such as SGLT2 inhibitors and mineralocorticoid receptor antagonists) and disease-modifying treatments (such as tafamidis) before irreversible multiorgan failure occurs. The study concludes that while the direct association between UACR and NAC stage did not reach conventional statistical significance ( $p=0.068$ ), the observed risk gradients and biological coherence support the utility of UACR for enhanced risk stratification in ATTR-CM.

Technical Summary: Cardiorenal Nexus in ATTR-Amyloidosis

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