Understanding the Role of Kidney Function Tests in Comprehensive Health Assessments

Kidney disease is the fastest-growing noncommunicable disease in the United States. Thirty-seven million Americans have kidney disease, millions more are at risk, and 90% of those with disease do not know they have it. Kidney disease is a silent killer, usually presenting without any signs or symptoms until it progresses to more advanced stages. Early and routine detection can save lives; utilizing various tests that are relatively noninvasive and inexpensive helps to catch and treat kidney dysfunction in its early stages to prevent the serious complications that can occur due to advanced kidney diseases, such as heart attack, stroke, kidney failure, and death. (26)

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Overview of Kidney Functions and Health

The kidneys are two bean-shaped organs located just below the ribcage on either side of the spine. The two kidneys play a crucial role in maintaining homeostasis within the body, serving as vital organs responsible for filtering waste products, excess fluids, and electrolytes from the blood to maintain fluid and acid-base balance in the body. The kidneys accomplish this with their complex network of microscopic structures called nephrons, the basic functional units responsible for blood filtration. Each nephron filters blood by removing wastes, excess fluids, and electrolytes to form urine and reabsorbing essential substances to maintain homeostasis. Additionally, the kidneys play an important role in vitamin D activation and make hormones that control blood pressure and red blood cell formation. (35)  

Chronic kidney disease (CKD), affecting more than 37 million American adults, is marked by the gradual loss of kidney function over time. When this happens, the kidneys lose their ability to perform their normal functions, leading to a buildup of wastes, electrolyte imbalances, anemia, and impaired bone health. Diabetes and high blood pressure are the two most common causes of CKD. (32) 

Acute kidney injury (AKI), often caused by sudden, severe infections, dehydration, blocked urine flow, or certain medications, is characterized by a rapid decline in kidney function within mere hours to days. This condition necessitates prompt medical attention to prevent further complications.

Other kidney-related health problems include polycystic kidney disease (PKD), kidney stones, and kidney infections. 

Types of Kidney Function Tests

Blood and urine kidney function tests serve as invaluable tools in assessing the health and functionality of the kidneys.

Estimated Glomerular Filtration Rate (eGFR)

eGFR is an estimation of GFR, a measurement of how well the kidneys filter blood. eGFR is calculated based on factors like serum creatinine (discussed below), age, sex, and race. eGFR is one of the mainstay biomarkers that represents kidney function, with a lower value signaling decreased filtration capacity. eGFR is one of two biomarkers that is used to stage CKD. An eGFR of less than 60 for at least three months may indicate an individual has kidney disease.

Blood Urea Nitrogen (BUN)

BUN measures the concentration of urea nitrogen in the blood. Urea nitrogen is a waste product formed in the liver during the breakdown of proteins. It is then carried to the kidneys, which filter it out of the blood and eliminate it from the body through urine. Impaired kidney function can cause BUN to build up; however, elevated levels can also indicate dehydration or high dietary protein intake.

Creatinine

Serum creatinine is a waste product derived from muscle metabolism. Like urea nitrogen, the kidneys filter and eliminate creatinine from the body. Elevated creatinine levels can be a marker of impaired renal function; however, high levels can also be caused by creatinine supplements, carnivorous diets, high-intensity exercise, and increased muscle mass. eGFR, BUN, and serum creatinine can be measured with the Comprehensive Metabolic Panel or Renal Function Panel by Access Med Labs.

Creatinine clearance is a test that uses blood and urine samples to quantify the amount of creatinine excreted over time, providing a direct measure of the kidneys' filtration rate. It offers an estimate of GFR and a more precise assessment of kidney function than serum creatinine alone by comparing the level of urine creatinine to serum creatinine. The Creatinine Clearance test by Doctor's Data is available to order through Rupa Health.

Cystatin C

Cystatin C is an alternative blood protein that can be used to calculate eGFR. Unlike creatinine, cystatin C is less influenced by muscle mass, offering a more accurate reflection of renal function. It is employed to complement traditional tests in assessing kidney health. You can measure Cystatin C with a blood test through BostonHeart Diagnostics.

Urinalysis

Urinalysis involves the comprehensive examination of urine, evaluating physical, chemical, and microscopic components. This test aids in detecting abnormalities such as blood, protein, or cellular elements, providing valuable diagnostic information for kidney-related conditions. A urinalysis with reflex to culture can be ordered through Access Med Labs.

Albumin-to-Creatinine Ratio

The urine albumin-to-creatinine ratio (UACR) assesses the levels of the protein albumin in relation to creatinine in urine. Elevated UACR indicates protein wasting, which is suggestive of kidney damage. This test is particularly relevant for detecting early signs of kidney disease before significant changes in eGFR can be detected. UACR is the second biomarker used in staging CKD; a UACR higher than 30 for three months or more is a sign an individual may have kidney disease. BostonHeart Diagnostics offers an Albumin/Creatinine Ratio (Urine) test on their test menu. (28)

Indications for Kidney Function Testing

Kidney function tests are performed to screen for, identify, and manage kidney disease. Lab results help doctors make informed decisions about appropriate treatment for patients with kidney disease and prevent further deterioration of kidney function.

Routine screening for kidney function is commonly ordered with a comprehensive metabolic panel (CMP) in asymptomatic, healthy patients during annual health screenings. Because BUN, creatinine, and eGFR (basic kidney tests included on a CMP) can be easily influenced by factors like age, supplements, medications, diet, hydration status, and muscle mass, additional tests, such as UACR and cystatin C may be advisable for certain populations, especially for individuals at higher risk for kidney disease. These risk factors include a personal history of hypertension, diabetes, cardiovascular disease, and obesity; a family history of kidney disease; age over 60; smoking; and frequent use of medications that can damage the kidneys, such as nonsteroidal inflammatory drugs (NSAIDs).

Symptomatic patients with signs of kidney pathology will require testing to confirm diagnosis and establish the underlying cause. Symptoms of kidney disease include persistent fatigue, swelling in the extremities, changes in urine color or frequency, dry and itchy skin, loss of appetite, and muscle cramping. 

Once a diagnosis of CKD has been established, the Kidney Disease: Improving Global Outcomes (KDIGO) guidelines recommend monitoring eGFR and urine albumin at least once annually. For patients with progressive, more severe CKD, these biomarkers should be monitored two to three times annually.

Interpreting Kidney Function Test Results

Elevated or reduced levels of certain substances in blood and urine can indicate impaired kidney function. Serum creatinine, BUN, cystatin C, and albumin have an inverse relationship to eGFR. In other words, with reduced GFR/kidney function, creatinine, BUN, albumin, and cystatin C tend to increase. On the other hand, eGFR and creatinine clearance are directly related to loss of kidney function – a reduction in one correlates to a reduction in the other. 

It's important to recognize that lab values are not isolated indicators; they must be interpreted in relation to one another and within the context of the patient's overall health and social history. Factors such as age, comorbid medical conditions, muscle mass, diet, hydration status, supplements, and medications can influence kidney markers, potentially leading to falsely high or low levels. (17)

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Managing and Monitoring Kidney Health

Prevention is key in managing kidney disease and preserving kidney function. Healthy lifestyle habits, which include adopting a nutritious diet, exercising regularly, staying hydrated, sleeping well, managing stress, limiting alcohol intake, and quitting smoking, play a pivotal role in maintaining optimal kidney function. Additionally, maintaining a healthy body weight and managing comorbid health conditions, such as diabetes, high blood pressure, and heart disease, will protect the kidneys from damage.

In addition to these kidney-essential foundations of health, complementary and integrative medical providers may also recommend adding kidney-specific herbal and dietary supplements to protect kidney function. Evidence supports the use of Cordyceps sinensis, Phyllanthus niruri, curcumin, silymarin (an extract of milk thistle seeds), probiotics, and vitamin D to preserve and support the kidneys in various kidney-related pathologies.

Regular follow-up and kidney monitoring are imperative for individuals with known kidney issues or a high risk of kidney disease. Routine check-ups should include kidney function tests, blood pressure and blood sugar measurements, and assessments of underlying conditions. The early identification of imbalances and underlying contributors to poor kidney function is imperative to prevent irreversible kidney damage and the associated consequences of reduced kidney function on whole-body health.

Advances in Kidney Function Testing

Recent advances and current research in kidney function testing have brought about innovative approaches, providing more accurate and nuanced insights into renal health. These developments contribute to a more comprehensive understanding of renal health, enabling healthcare professionals to make informed decisions for personalized and effective patient care.

Technological advancements have led to the introduction of point-of-care testing devices for kidney function, allowing for quicker and more convenient assessments. These devices offer rapid results, enabling early diagnosis and timely interventions in various healthcare settings.

Integrating artificial intelligence (AI) and machine learning algorithms has enhanced the interpretation of kidney function data. These technologies can analyze large datasets, identify patterns, and predict changes in kidney function, contributing to personalized, predictive, and preventive medicine. AI in nephrology promises to revolutionize the early detection of kidney disease, enhance diagnostic accuracy, and improve patient outcomes.

Moreover, ongoing research is working to identify novel biomarkers associated with specific kidney conditions to better understand disease progression and prognosis. Discovering and developing sensitive and specific biomarkers would allow for the prediction and diagnosis of kidney dysfunction and CKD complications in their early stages so that they could be more easily reversed or prevented altogether. 

The continued advancement of kidney disease testing, coupled with early screening initiatives, has the potential to reduce poor health outcomes significantly. According to a study assessing the costs and implications of kidney disease screening, researchers found that screening adults aged 35 to 75 years for CKD could prevent the necessity for dialysis or kidney transplant in 398,000 to 658,000 individuals over their lifetime, contingent on the frequency of screening.

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Kidney Function Testing: Key Takeaways

Kidney function tests are indispensable tools in comprehensive health assessments, playing a pivotal role in the early detection and management of kidney diseases. These tests, encompassing markers such as eGFR, serum creatinine, BUN, and urine albumin, provide crucial insights into the intricate filtration processes of the kidneys. By employing regular laboratory assessments, individuals can proactively address potential issues related to kidney function, allowing for early interventions and personalized management strategies. The recent advances in kidney testing, including the integration of AI, point-of-care devices, and innovative biomarker discovery, further underscore the dynamic landscape of renal health assessment. As we navigate this evolving terrain, fostering awareness and promoting regular monitoring emerge as paramount practices to ensure optimal kidney health, empowering individuals to take charge of their well-being and mitigate the impact of kidney diseases on long-term health outcomes.