Glomerular Filtration Rate, Albuminuria, and Reported Kidney Disease in Comparison

Results From the German National Cohort (NAKO)

Background: Chronic kidney disease (CKD) can be asymptomatic for many years and is often diagnosed late. Given the availability of new treatments, the early identification of relevant findings from screening of the kidney markers estimated glomerular filtration rate (eGFR) and albuminuria in the general population is becoming increasingly important.

Methods: In the NAKO study, self-reported medical diagnoses of kidney disease in 195 182 participants were compared with relevant findings from screening biomarkers (eGFR < 60 mL/min/1.73 m² and albuminuria). For the purpose of comparison, various equations for assessing kidney function were evaluated as well.

Results: 2% of the participants reported having received a medical diagnosis of kidney disease, and 2% had an eGFR below 60 mL/min/1.73 m². There was, however, little overlap between these two groups: more than 80% of participants with an eGFR between 30 and 59 mL/min/1,73 m² did not report any diagnosis of kidney disease. The additional inclusion of data on albuminuria did not materially affect this discrepancy: 6213 persons (17.5% of the cohort) with an abnormal eGFR or urinary albumin-to-creatinine ratio (UACR) did not report any diagnosis of kidney disease. Even among participants whose eGFR was in the range of 30–59 mL/min/1.73 m² and whose UACR was above 300 mg/g, less than half reported having a medically diagnosed kidney disease.

Conclusion: These findings indicate a low level of awareness regarding the possible presence of CKD in the general population. Many people with abnormal screening findings needing further investigation due to their potential clinical relevance are unaware that they might be suffering from a kidney disease. As more effective treatments for kidney disease are now available, these findings indicate a need for structured screening and evaluation strategies to promote kidney health.

Cite this as: Sekula P, Butz E, Schaeffner E, Nolde JM, Schmidt IM, Brackmann LK, Fischer B, Girndt M, Günther K, Hannemann A, Harth V, Heinsohn T, Karch A, Keil T, Krist L, Lange B, Leitzmann M, Meinke-Franze C, Meister J, Mikolajczyk R, Mons U, Nimptsch K, Obi N, Övermöhle C, Pischon T, Schikowski T, Schöttker B, Schulze MB, Schwichtenberg J, Stang A, Teismann H, Völzke H, Endlich K, Nauck M, Scholz M, Heid IM, Lieb W, Köttgen A: Glomerular filtration rate, albuminuria, and reported kidney disease in comparison: Results from the German National Cohort (NAKO). Dtsch Arztebl Int 2026; 123: 345–52. DOI: 10.3238/arztebl.m2026.0053

Hundreds of millions of people worldwide live with chronic kidney disease (CKD) (1, 2, 3). The prevalence of CKD continues to rise sharply and it is now ranked among the ten leading causes of death (1, 4). New WHO guidelines recognize kidney health as a growing global priority (5); the number of deaths caused by CKD is expected to further increase in the future (6).

Many people with CKD are unaware of their disorder, because CKD is typically asymptomatic in its early stages and thus goes undiagnosed initially (2, 7). Various studies have in fact shown that a large proportion of people with CKD, including those in high-risk groups, are not formally diagnosed with CKD (8, 9, 10, 11, 12, 13, 14, 15, 16, 17). With a paradigm shift in CKD management underway, early detection is now more important than ever: Slowing disease progression was long viewed as the only realistic management strategy, until recent randomized trials and meta-analyses have shown that, with modern combination therapy and in selected patient populations, remission is actually achievable (18, 19).

In this light, early diagnosis of CKD is becoming ever more important, so that people affected can benefit from the therapeutic potential of modern treatment strategies (8, 9, 20, 21). In routine clinical practice, the markers most commonly used to diagnose kidney disease include the estimated glomerular filtration rate (eGFR) for assessing kidney function and albumin in the urine for identifying kidney damage (2). Equations commonly used to estimate GFR rely on serum measurements of creatinine and/or cystatin C and are selected considering age and the intended use, amongst other things (2, 22, 23). Most of the currently available information on the need for expanded screening for CKD originates from population-based surveys, databases maintained by treating physicians, and prospective cohort studies (20). There is also various data on CKD available from German studies with up to about 10 000 participants—from regional studies and studies focusing on older adults (3, 17, 24, 25, 26, 27).

Given Germany’s aging population and the importance of CKD as a widespread disease, nationwide data are highly relevant. Large, population-based cohort studies with biospecimen collection and long-term follow-up—such as the German National Cohort (NAKO) (https://nako.de) with over 200 000 participants—today represent a key research infrastructure, offering adequate statistical power (28).

The primary aim of our analysis was to determine, based on cross-sectional data from the NAKO study, the number of participants with screening findings that would require further diagnostic testing to rule out CKD (eSupplement Figure 1). In particular, the reported diagnoses were to be compared with the available laboratory results for eGFR and albuminuria in order to establish how many persons have abnormal laboratory findings (eGFR < 60 mL/min/1.73 m² or UACR ≥ 30 mg/g), yet are unaware of potentially having kidney disease. The second aim was to compare different equations for calculating eGFR.

Methods

Study population and data

The German National Cohort (NAKO study) is a prospective, population-based cohort study (DRKS00037328) conducted in Germany (28). Between 2014 and 2019, a total of 205 415 adults aged 19 to 74 years (overall response rate: 15.6%) were included in the study (29). Approvals from the relevant ethics committees and written informed consent from the participants were obtained (30).

As part of collecting information on pre-existing conditions, the following question was asked: “Has a doctor ever diagnosed you with impaired kidney function or chronic kidney disease?“ (eSupplement Table 1) (31). Further questions concerned information about dialysis treatment and about a previous kidney transplant. In this project, a reported kidney disease was considered to be present if the response to one of these three questions was “yes.” Dialysis treatment or a previous kidney transplant was combined into the category of “kidney replacement therapy” (KRT).

Based on available serum measurements of creatinine (96%) and cystatin C (55%) in the study population, eGFR was calculated using various estimation equations (eSupplement Table 2). The primarily used equation in this study was the creatinine-based European Kidney Function Consortium equation according to Pottel et al. (32).

The urinary albumin-to-creatinine ratio (UACR) as an indicator of kidney damage was determined using urine test strips (Siemens Clinitek) specifically designed to detect microalbuminuria. The categories recorded (<30, 30–300 [microalbuminuria] and >300 mg/g [macroalbuminuria] correspond to the KDIGO risk classification for kidney damage (A1–A3) (2). KDIGO risk categories were also used to classify eGFR values: ≥ 60 (G1–2: normal to mildly impaired), 30–59 (G3: mildly to moderately impaired), and < 30 mL/min/1.73 m² (G4–5: severely impaired/kidney failure) (2). Impaired kidney function was defined as an eGFR < 60 mL/min/1.73 m².

Statistical analysis

Based on a data export from September 2023 (N = 204 770), complete data on reported kidney disease and creatinine levels were available from 195 182 participants (eSupplement Figure 1). Additional information on albuminuria was available for 35 504 participants (18%) and information on cystatin C levels was available for 112 151 participants (57%). Given that KRT affects the interpretability of kidney function parameters, participants receiving KRT were excluded from figures and analyses related to eGFR and UACR.

Apart from unweighted information on means and proportions, weighted values were also calculated using the NAKO study weights to better describe the composition of the underlying population (29, 33, 34). The weighted statistics take into account that the probability of being selected for the study varied between participants depending on their age and sex, and that selected persons with specific characteristics more frequently consented to participation than others (29, 35).

GFR estimation equations were compared using pairwise concordance correlation coefficients (36). Logistic regression models were used to identify factors associated with non-reporting of a kidney disease.

The statistical software R version >4.0.5 was used for all statistical analyses.

Results

The mean age in the study population was 50 ± 13 years, with an equal sex distribution (Table 1). Of the participants, 41.2% had hypertension and 6.8% had diabetes mellitus.

Frequency of reported kidney disease and of eGFR < 60 mL/min/1.73 m²

A medically diagnosed kidney disease was reported by 2.2% (weighted: 2.1%) of the participants. This proportion increased, as expected, with age, from 0.8% among 19– to 29-year-olds to 3.5% among 60– to 75-year-olds (eSupplement Table 3). A total of 227 participants (0.1 %) reported to have KRT (Table 1).

Table 1

Description of the study population

Among the 194 955 participants not having KRT, the mean eGFR was 91 ± 15 mL/min/1.73 m² (weighted: 94 ± 15), which was within the reference range and reflected the study design (Table 1). The proportion of participants with an eGFR < 60 mL/min/1.73 m² increased from 0.1% among those aged 19 to 29 years to 7.8% among those aged 60 to 75 years (overall: 2.6%; weighted: 2.2%; eSupplement Table 3). The results were confirmed in a sensitivity analysis, testing the creatinine-based results for possible batch effects (eSupplement Methods 1).

Concordance between reported kidney disease and abnormal laboratory test results

Of the participants without KRT, 8201 reported having been diagnosed with kidney disease or had an eGFR < 60 mL/min/1.73 m². However, both criteria were met in only 875 participants. The subcohort with urine test data (N = 35 504; without KRT, N = 35 461) was used to investigate the low concordance in more detail, taking into account the possible presence of kidney damage in persons with a normal eGFR. The characteristics of the subcohort were very similar to those of the overall cohort (Table 1) and the subgroup had virtually identical weighted percentages of reported kidney disease (2.3%) and eGFR < 60 mL/min/1.73 m² (2.2%). Microalbuminuria (UACR 30–300 mg/g) was present in 15.9 % (weighted: 16.6%) of participants and macroalbuminuria (UACR > 300 mg/g) in 0.8% (weighted: 0.8%).

Figure 1 illustrates the concordance of reported kidney disease, eGFR < 60 mL/min/1.73 m² and UACR ≥ 30 mg/g in participants without KRT. While at least one of the three criteria applied to 6 993 participants, only 304 out of 35 461 persons (0.9%) reported a kidney disease that was in line with the screening findings. A total of 6213 persons (17.5%) reported no kidney disease despite abnormal eGFR or UACR values. Notably, in the KDIGO risk category G3 (30–59 mL/min/1.73 m²), the proportion of reported kidney disease across categories A1 through A3 was considerably below expectations (Table 2). Even in persons with an eGFR of 30–59 mL/min/1.73 m² and an UACR > 300 mg/g, this proportion was less than 50%. Women (Figure 2) and older participants (eSupplement Figure 2) very often reported no kidney disease despite abnormal findings.

Figure 1

Group sizes and overlap among NAKO participants with regard to all combinations of reported kidney disease

Figure 2

Sex-specific numbers and proportions of NAKO participants in KDIGO risk categories with abnormal eGFR or UACR values who reported having kidney disease or no kidney disease

Table 2

Percentage of persons with reported kidney disease in the respective KDIGO risk category

Among participants with an eGFR < 60 mL/min/1.73 m² or a UACR ≥ 30 mg/g (N = 1,032), increasing age was associated with a higher frequency of participants reporting no kidney disease (odds ratio [OR]: 1.29 per 10 years; 95% confidence interval: [1.05; 1.57]; eSupplement Table 4). On the other hand, men (OR 0.64; [0.44; 0.91]) and participants with an eGFR < 30 mL/min/1.73 m² (OR 0.01; [0.00; 0.04]) or a UACR ≥ 30 mg/g (OR 0.42; [0.28; 0.62]) reported less frequently that they had not received such a diagnosis compared to their comparison groups. Similarly, the probability of not being diagnosed with kidney disease was lower in participants with diabetes mellitus (OR 0.7; [0.44; 1.01]) or hypertension (OR 0.65; [0.4; 1.03]) compared to participants without these diseases; however; these differences were not significant (eSupplement Table 4).

Comparability of eGFR calculations

The evaluation of eight different estimation equations in the subcohort with cystatin-C measurements (N = 112 151; eSupplement Tables 2 and 5) revealed differences in a given person’s eGFR values, amounting to a median of 17.0 mL/min/1.73 m² (Q3: 22.1). The concordance correlation coefficients were good to excellent for equations that used the same biomarker(s) (range: 0.74–0.97) and less good when comparing creatinine-based equations with cystatin C-based equations (range: 0.53–0.66; eSupplement Figure 3). The estimates differed, in some cases systematically, particularly in the range of higher eGFR values (eSupplement Figure 4).

The means for eGFR and the respective proportion of participants with eGFR < 60 mL/min/1.73 m² ranged between 91.4 ± 14.3 (weighted: 94.2 ± 14.7) and 2.3% (weighted: 2.0%) according to Pottel et al. (32) and 104.1 ± 15.4 (weighted: 106.6 ± 16.2) mL/min/1.73 m² and 0.9% (weighted: 0.9%) according to Inker et al. (22) (Table 3). However, the lack of concordance between reported kidney disease and abnormal laboratory findings was observed regardless of the GFR estimation equation used (eSupplement Figure 5).

Table 3

Mean eGFR values and proportion of NAKO participants with eGFR values < 60 mL/min/1.73 m2 by estimation equation

Discussion

The aim of this study was to generate previously missing information on the occurrence of abnormal laboratory results for kidney function parameters and reported diagnosed kidney disease, based on data from the NAKO study which recruited participants from across Germany (28). The analysis reveals clear evidence of a relevant gap in awareness that CKD may be present.

In the overall cohort of 195 182 participants (without KRT, N = 194 955), weighted 2.1% reported a medically diagnosed kidney disease. The weighted proportion of participants with an eGFR < 60 mL/min/1.73 m² was very similar (2.2%). In the cohort with urine test data (N = 35 504; without KRT, N = 35 461), weighted 2.3% reported a kidney disease. Weighted 2.2% had an eGFR < 60 mL/min/1.73 m² and weighted 17.4% had a UACR ≥ 30 mg/g.

Three German studies reported CKD prevalence rates of 11.2%, 12.7% and 17.3%, respectively, and 2.2%, 2.3% and 5.9%, respectively, of those affected had an eGFR < 60 mL/min/1.73 m² (3, 24, 27). These figures are similar to the NAKO findings observed in our study and are based on a similar age and sex distribution, with also one laboratory value for eGFR or UACR in each study. Minor discrepancies are likely due to differences in the sampling method, the sample size or the methods used.

The reported prevalence rates for CKD vary significantly between countries (3, 4, 16). Taking into account the age and sex distributions in Europe, the proportion of persons with an eGFR < 60 mL/min/1.73 m² ranges from 1% in Italy to 5.9% in northeastern Germany, and when albuminuria is also taken into account, the proportion ranges from 3.3% in Norway to 17.3% in northeastern Germany (3). This variation can likely be attributed to both methodological differences (e.g., selection of the study population, measurement methods) and regional differences in lifestyles, risk factors and healthcare (3).

As expected, the proportion of NAKO participants who reported kidney disease as well as those with an eGFR < 60 mL/min/1.73 m² and a UACR ≥ 30 mg/g increased with age (3, 24, 37). Using a comparable eGFR equation, the Berlin Initiative Study (BIS) reported an eGFR-based CKD prevalence of 38% among people aged 70 and older (25), a finding that supports our observation.

In the NAKO study, the overlap between a reported kidney disease and abnormal laboratory test results was small. The proportion of participants without abnormal laboratory findings who nevertheless reported medically diagnosed impaired kidney function could, for example, be explained by transient kidney dysfunction, diagnoses based on criteria not covered in this study, biological and analytical variability in laboratory parameters, and misunderstandings in the communication of medical findings. Of particular note is that only a minority of NAKO participants with objectively abnormal laboratory test results reported ever having been diagnosed with kidney disease.

Given the lack of repeat testing, this cross-sectional study does not allow for a formal diagnosis of CKD. Nevertheless, it can be assumed that, besides persons with transiently abnormal findings, there is a significant subgroup in which the observed abnormalities would be confirmed upon repeat testing. Earlier population-based studies have consistently shown that, among a significant proportion of people with abnormal screening results, an abnormal eGFR or albuminuria finding is confirmed when the test is repeated (27, 38, 39). Depending on the study population, the parameter examined and the time interval until the follow-up measurement, approximately 40–60% of individuals with an initial eGFR < 60 mL/min/1.73 m² showed persistent abnormalities, with some cohorts reporting even higher confirmation rates of up to approximately 70–80% (38, 39).

Based on these data, it appears that a structured, population-wide implementation of CKD screening strategies is needed to close any potential diagnostic gaps as early as possible. In the event of abnormal initial findings, repeat testing should be performed in a timely manner and consistently, so that a diagnosis of CKD can be established in accordance with the guidelines. While the aim of CKD treatment has long been to slow the loss of kidney function, recent randomized trials and meta-analyses show that modern combination therapy can stabilize eGFR courses within the range of normal age-related decline and substantially reduce or even normalize albuminuria (18, 19). With these advances, the concept of CKD remission is moving increasingly closer to reality. However, in order to achieve this, affected persons must be identified at an early stage, before irreversible structural kidney damage has occurred. With that in mind, it is necessary to close potential diagnostic gaps, not only from an epidemiological viewpoint, but increasingly also from a prognosis and health economics perspective. The figures reported in the international literature regarding the lack of concordance are well aligned with the findings of our study (2, 14, 24). Similar to what we observed in our study, Tangri et al. reported associations between a lack of awareness of having CKD and older age, female sex, and persons without diabetes mellitus or hypertension (14).

The discrepancy between reported kidney disease and an eGFR < 60 mL/min/1.73 m² could also be attributable to the estimation of GFR, which can lead to both false-positive diagnoses and CKD being overlooked (26, 40). Although the choice of equation is important both for the individual patient and for determining the prevalence of people with reduced kidney function, the lack of concordance between self-reported kidney disease and abnormal laboratory test results was observed regardless of the estimation equation used.

The strengths of our study include its size and wide geographical coverage as well as the availability of data on creatinine and cystatin-C levels and albuminuria. In addition, standardized instruments were used for both the data collection and the measurements. We were able to include study weights that adjust the results for differences in selection probabilities and selective nonresponse in the study population. The data reported here should not be interpreted as a representative prevalence estimate for the general population of Germany. The NAKO study intentionally recruited adults between the ages of 19 and 74 years from predefined regions to allow for long-term prospective observation. The reported proportions thus apply to these age groups within the study regions and cannot, for example, be extrapolated to very elderly persons. Another limitation is the lack of timely repeat measurements, which are impractical in observational studies, making it impossible to formally diagnose CKD.

Conclusion

In the large German NAKO study, approximately 2% of participants reported a kidney disease. There was, however, little overlap with abnormal eGFR or UACR values. This finding illustrates a lack of awareness of the possibility of having CKD among the general population. The onset of CKD is insidious. The key to better early detection is therefore to raise awareness of this problem among patients and physicians and to consistently follow up on abnormal screening results with repeat measurements.

Additional authors
Elke Schaeffner, Janis M. Nolde, Insa M. Schmidt, Lara Kim Brackmann, Beate Fischer, Matthias Girndt, Kathrin Günther, Anke Hannemann, Volker Harth, Torben Heinsohn, André Karch, Thomas Keil, Lilian Krist, Berit Lange, Michael Leitzmann, Claudia Meinke-Franze, Jaroslawna Meister, Rafael Mikolajczyk, Ute Mons, Katharina Nimptsch, Nadia Obi, Cara Övermöhle, Tobias Pischon, Tamara Schikowski, Ben Schöttker, Matthias B. Schulze, Julia Schwichtenberg, Andreas Stang, Henning Teismann, Henry Völzke, Karlhans Endlich, Matthias Nauck, Markus Scholz, Iris M. Heid, Wolfgang Lieb

Data sharing
The data of the NAKO study are made available to researchers with approved research applications. More information on data requests can be found here: https://transfer.nako.de/transfer/index

Acknowledgement
This analysis was conducted with data from the German National Cohort (NAKO) study (www.nako.de). The NAKO study has been funded by the German Federal Ministry of Research, Technology and Space (BMFTR, Bundesministerium für Forschung, Technologie und Raumfahrt) (grant numbers: 01ER1301A/B/C, 01ER1511D, 01ER1801A/B/C/D and 01ER2301A/B/C), the Federal States (“Länder”) and the Helmholtz Association, as well as the participating universities and institutes of the Leibniz Association. We thank all participants and staff members of the NAKO study.

Funding
The work by PS and AK was funded by the German Research Foundation (DFG, Deutsche Forschungsgemeinschaft) through SPP 2177 (3598/6–2) and DFG project ID431984000 (SFB 1453). PS received funding from the DFG to support the work of EB (SE 2407/3–1). IMH was funded by TRR 374 TP C6 (Project ID: 509149993). JMN receives funding from the Berta-Ottenstein program for Clinician Scientists of the University Freiburg.

Conflict of interest statement
EB received travel expense support from DGfN.

AKa is the President of the German Society for Epidemiology (DGEpi).

RM is a member of the board of the German Society for Epidemiology.

ES receives consulting fees from AstraZeneca and a fee from the National Kidney Foundation for editorial work at the American Journal of Kidney Diseases. She is a spokesperson of the European Kidney Function Consortiums and was a member of the working group of the KDIGO Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. She is the vice president of the German Society of Nephrology (DGfN).

MS states ongoing collaboration with Owkin on the topic of heart failure.

KE is co-inventor and holds a patent (EP3396380B1) for a diagnostic instrument used to identify podocyte foot-process effacement. He is the treasurer of the Förderverein Nordverbund Niere.

TP and WL are volunteer members of the board of NAKO e.V. which oversees the NAKO study.

MG is a member of the extended board of the German Society of Nephrology and Subject Editor for the Clinical Kidney Journal. He has received lecture fees from Boehringer-Ingelheim, AstraZeneca, Novartis, and Glaxo Smith Kline.

The remaining authors declare no conflict of interest.

Manuscript received on 30 July 2025; revised version accepted on 24 March 2026

Translated from the original German by Ralf Thoene, M.D.

Corresponding author
PD Dr. Peggy Sekula

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