New Study Suggests Potential of penKid in Kidney Reserve Assessment

• A new peer-reviewed study in Physiological Reports is the first to evaluate Proenkephalin A 119-159 (penKid) for kidney functional reserve (KFR) assessment, a stress test designed to reveal loss of kidney capacity that may not be apparent from routine blood tests.
• Within that method, penKid stood out: its dynamic had a far wider range after a protein stimulus than cystatin C (44.2% vs. 10.5% median fractional change) while reaching its maximal response within approx. two hours after protein stimulation.
• The results extend the published evidence for penKid beyond acute kidney injury into a potential complementary application: assessing kidney functional reserve.

Hennigsdorf, Germany – October 6, 2026 – penKid has been evaluated for the first time as a marker of kidney functional reserve (KFR): a new study in Physiological Reports (1) found that its response spanned a much wider range than cystatin C and creatinine clearance after a protein stimulus. The study in 62 participants marks the first published evaluation of penKid for this use.
Clinical context and unmet need
Kidney function may appear normal on a standard blood test until a large portion of capacity has already gone. Kidney functional reserve testing addresses this gap by measuring how much a kidney's filtration rate can still rise in response to a stimulus, most commonly a protein meal. Once that reserve narrows, the kidney has less room to compensate for future stress such as major surgeries.  The study authors note that reserve testing is rarely used in clinical practice, largely because the reference method used, creatinine clearance, is cumbersome, requiring blood and urine collection, can be difficult to perform outside specialized settings, and is affected by processes in the kidney unrelated to filtration itself.
Study design and key findings
Researchers at the University of New South Wales and Prince of Wales Hospital in Sydney conducted the study in 62 participants: 40 patients having heart surgery, and 22 healthy people, including volunteers, kidney donors, and patients who had a kidney auto-transplant. Participants drank a protein shake, followed by hourly collection of blood samples over a five hour-period. Urine samples were also collected for creatinine clearance testing.
After a high protein intake, kidney filtration increases for a few hours: this "extra work" is what the methods were designed to measure. PenKid showed the largest dynamic response, with a median fractional increase of 44% from baseline compared with 30% for creatinine clearance and only about 10% for cystatin C. Unlike creatinine clearance, penKid differentiated healthy participants from cardiac surgery patients, who had a lower KFR. The study also found penKid measurements to be more homogeneous than creatinine clearance measurements, indicating greater consistency across assessments.
PenKid was also fast, reaching its peak functional response in about two hours, similar in timing to a two-hour diabetes test (oral glucose tolerance test).
Implications for clinical practice
Beyond its wider dynamic range, the authors highlight practical advantages specific to penKid-based functional reserve testing: no urine collection is needed, minimal fasting is required, and penKid enables near real-time assessment of kidney functional reserve from blood samples alone. These features suggest penKid may offer a fast and simple alternative to assess how much reserve an individual patient has.
"PenKid gave us a fast, consistent read on how much filtration capacity a kidney still has in reserve," said Prof. Zoltan H. Endre, corresponding author of the study, Department of Nephrology, Prince of Wales Hospital, and School of Clinical Medicine, University of New South Wales, Sydney. "A signal that clear, captured that quickly, turns a kidney stress test from a research tool into something a clinic could actually use."
About penKid, scientific insights
PenKid is a biomarker for real-time assessment of kidney function. Unlike creatinine, penKid levels are not influenced by inflammation or factors such as age or sex, and prior studies show penKid can detect acute kidney injury (AKI) up to 48 hours before conventional criteria are met (2,3). The new Physiological Reports study provides data for another, complementary application: measuring kidney functional reserve to identify subclinical kidney disease.
About SphingoTec
SphingoTec GmbH ("SphingoTec"; Hennigsdorf near Berlin, Germany) is a biomarker company focusing on the out-licensing of innovative critical care solutions for diagnosing, predicting, and monitoring acute medical conditions. SphingoTec develops its biomarkers to the commercial stage and partners with IVD companies to make them available on different IVD platforms. SphingoTec's proprietary biomarker portfolio includes Proenkephalin A 119-159 (penKid), a biomarker for the assessment of kidney function in critical diseases, and bioactive Adrenomedullin 1-52 (bio-ADM), a biomarker for the assessment of endothelial function in conditions like acute heart failure. Discover more on www.sphingotec.com
References

1. Rodrigues, C. E. et. al. (2026). Plasma proenkephalin-a facilitates measurement of human kidney functional reserve: From healthy kidney donors to polymorbid patients. Physiological Reports, 14, e71083. https://doi.org/10.14814/phy2.71083.
2. Caironi  P, et al. (2018) Circulating Proenkephalin, Acute Kidney Injury, and Its Improvement in Patients with Severe Sepsis or Shock.; https://doi.org/10.1373/clinchem.2018.288068
3. Rosenqvist M, et. al. (2019) Proenkephalin a 119-159 (penKid) - a novel biomarker for acute kidney injury in sepsis: an observational study. BMC Emerg Med, doi: 10.1186/s12873-019-0283-9.

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