SHARE blood tests find millions are living with undiagnosed kidney disease

85% of cases remain undetected despite routine doctor visits.

Understanding chronic kidney disease: why early detection matters

(January 2026) The human kidneys form your body’s filtration system – they’re two fist-sized organs that work hard to remove waste and extra water from the body through cleaning your blood and creating urine (processing up to 50 gallons a day!). Without this constant filtration, toxins would build up in your bloodstream and make you sick. Kidneys also keep all the important substances like sodium, potassium, and calcium in your blood perfectly balanced and, finally, they help make red blood cells.

Kidney function tends to decline with age and may develop into chronic kidney disease (CKD). CKD is a huge global health burden, affecting nearly 850 million people worldwide and about 100 million Europeans. Despite the initiatives of the KDIGO (Kidney Disease – Improving Global Outcomes) organisation, public awareness of the disorder is on the low side, while the case count continues to rise. For that reason, early detection of CKD warning signals is crucial to delay the onset of the disease for as long as possible.

People in advanced stages of the disease with very low kidney function need machines to clean their blood artificially (dialysis), or once the kidney fails, an organ transplant to survive. As kidney replacement therapy is very expensive, CKD poses also a huge burden for health care systems (an estimated 150 billion euros annually only in Europe).

Moderate to severe CKD already takes a serious toll on people's lives, affecting their quality of life and increasing their risk of dying earlier. These outcomes are made worse by other illnesses which tend to show up around the same time, like diabetes, high blood pressure, heart disease, stroke, and heart failure. In fact, nearly everyone with later-stage CKD will have other illnesses too. high blood pressure and high blood sugar damaging the kidneys, once these conditions are present they often interact and worsen one another.

However, it’s nearly impossible to intervene when the vast majority of cases go undiagnosed. The estimated rate is an astonishing 70% in Europe, probably because the early-stage disease has few symptoms.

Using dried blood spot sampling for large-scale screening of chronic kidney disease

Researchers Hannah Marie Horton and Martina Börsch-Supan from the SHARE biomarker team at the Munich Research Institute for the Economics of Aging and SHARE Analyses (MEA) set out to understand this detection gap using data from the Survey of Health, Ageing and Retirement in Europe (SHARE). In their study published in PLOS One and funded by the National Institute of Aging, they examined blood samples from over 22,000 adults aged 50 and older across 12 countries to identify people with kidney dysfunction. They then compared these laboratory results with whether people actually reported having been diagnosed with CKD by a doctor.

To identify kidney disease, the researchers analysed blood collected by trained interviewers as dried blood spot (DBS) samples on filter paper cards by a simple finger prick during the interview in SHARE Wave 6 (2015). Rather than requiring participants to visit a clinic for traditional blood draws, trained interviewers collected small blood samples on filter paper cards during home visits. These samples were then analysed for cystatin C – a protein that builds up in the blood when kidneys aren't filtering properly. This minimally invasive approach made it possible to screen thousands of people who might never have had their kidney function tested otherwise.

The big result: 85% of kidney disease cases go undetected in older adults

About 11% of older Europeans in the SHARE sample were living with chronic kidney disease without knowing it. Even more striking, among all those who actually have the condition based on the blood test, approximately 85% remain undiagnosed.

Diabetes and hypertension don't trigger kidney disease screening as expected

The researchers also investigated whether having other health conditions made it more or less likely that someone's kidney disease would be caught. As previously mentioned, conditions like diabetes and high blood pressure are well-known risk factors for kidney damage – so you might expect that people with these conditions would be routinely screened for kidney problems.

Surprisingly, the study found this isn't happening consistently. Even people with four or more health conditions faced only a 27% chance of having their kidney disease detected.

Instead, CKD diagnoses are more likely to occur in people who are receiving intensive care for conditions like cancer or arthritis, which require frequent medical visits and monitoring. While these conditions are related to CKD as well, doctors should rather be prompted by the CKD heavy-hitters (like diabetes or hypertension) to screen for the disease.

Women and older adults most likely to have undiagnosed kidney disease

The study revealed troubling disparities. Women and older individuals are significantly more likely to remain undiagnosed, even when their kidney function has declined substantially. Those over 75 have less than an 18% chance of detection.

European countries show surprising variations in kidney disease detection

The prevalence of undiagnosed CKD varied considerably across Europe, with countries like Sweden and Switzerland showing unexpectedly high rates (around 14-15%) despite their well-funded. In these countries, healthcare is organised regionally rather than nationally, which may affect how consistently kidney disease is detected. Meanwhile, countries like Italy, Greece, and Estonia appeared to catch more cases earlier, though overall detection rates remain disappointingly low everywhere.

These differences don't seem to follow a simple pattern based on healthcare spending or system type. Instead, they likely reflect how actively different countries screen for kidney disease, whether doctors routinely check both kidney function and protein (albumin) levels in urine, and how well general practitioners and kidney specialists work together.

Why SHARE data made this research possible

This study overcomes two major challenges that have limited previous research on undiagnosed kidney disease:

Reaching people where they are: Most studies on kidney disease only capture people who regularly visit doctors or clinics. This creates a skewed picture because those who rarely seek medical care – often the very people with undiagnosed conditions – never get counted. SHARE's approach of visiting people in their homes meant researchers could include a much more representative sample of older adults, capturing those who might otherwise remain invisible to the healthcare system.

Comparing apples to apples: When trying to understand differences between countries, it's crucial that everyone is measured the same way. SHARE collected dried blood spot samples from all participants, which were then analysed in a single centralised laboratory using identical methods. This standardisation is rare and invaluable: it means that when researchers see differences between countries, they can be confident these reflect real variations in health and healthcare, not just differences in how measurements were taken.

What healthcare systems need to do now

It’s clear that having chronic conditions should trigger more systematic screening for kidney disease. The fact that even people with four or more health conditions have less than a 30% chance of kidney disease detection suggests current screening practices aren't working.

Early detection matters enormously. While kidney disease can't usually be cured, catching it early allows doctors to manage the underlying causes – like controlling blood pressure or blood sugar – and slow the disease's progression. Late diagnosis limits treatment options and increases the risk of serious complications.

As populations age across Europe, understanding and addressing undiagnosed chronic diseases becomes increasingly urgent for both individual health and healthcare systems facing mounting pressure.

Study by Horton HM, Börsch-Supan M (2025) High unawareness of kidney dysfunction in European older adults and the importance of early detection through comorbidities. PLoS One 20(10): e0333578.

URL: https://doi.org/10.1371/journal.pone.0333578