Hypertension is a chronic medical condition where the blood pressure in the arteries is persistently elevated. It is a significant risk factor for cardiovascular diseases, stroke, and kidney disease.
Monitoring kidney health in hypertensive patients is crucial because the kidneys play a pivotal role in regulating blood pressure and maintaining overall fluid and electrolyte balance. Untreated hypertension can lead to kidney damage, creating a detrimental cycle that exacerbates both conditions.
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Understanding Hypertension
According to the American College of Cardiology/American Heart Association (ACC/AHA) 2017 guidelines, hypertension is defined as having a systolic blood pressure (SBP) of 130 mmHg or higher, or a diastolic blood pressure (DBP) of 80 mmHg or higher.
Blood pressure categories are as follows:
- Normal: < 120/80 mmHg
- Elevated: SBP 120-129 mmHg and DBP <80 mmHg
- Stage 1 Hypertension: SBP 130-139 mmHg or DBP 80-89 mmHg
- Stage 2 Hypertension: SBP ≥140 mmHg or DBP ≥90 mmHg
Hypertension affects approximately 1.28 billion adults globally. The global burden of hypertension contributes significantly to morbidity and mortality, underscoring the need for effective management strategies.
Types of Hypertension
Hypertension can be classified into primary (essential) and secondary types:
- Primary hypertension accounts for about 90-95% of cases and has no identifiable cause but is associated with genetic, lifestyle, and environmental factors.
- Secondary hypertension results from an underlying condition, such as kidney disease, sleep apnea, adrenal disorders, thyroid disease, or certain medications.
Hypertension Risk Factors
Key risk factors for high blood pressure include:
- Genetic Predisposition: family history of hypertension, male sex, race (Black, Hispanic, and Asian)
- Lifestyle Factors: high salt intake, obesity, physical inactivity, excessive alcohol consumption, smoking, stress
- Comorbid Medical Conditions: diabetes, high cholesterol, sleep apnea, chronic kidney disease (CKD)
What Is the Relationship Between Blood Pressure and the Kidneys?
The kidneys help regulate blood pressure through the renin-angiotensin-aldosterone system (RAAS), which controls blood volume and systemic vascular resistance.
- When blood pressure drops, the kidneys secrete a hormone called renin.
- Renin converts angiotensinogen into angiotensin I, which is then transformed into angiotensin II.
- Angiotensin II causes the constriction (narrowing) of blood vessels and stimulates the release of aldosterone from the adrenal glands.
- Aldosterone promotes sodium and water retention by the kidneys, increasing blood volume and raising blood pressure.
- As blood pressure rises, the kidneys reduce renin secretion, creating a feedback loop that helps maintain stable blood pressure levels.
Renal hypertension is a type of secondary high blood pressure that arises from the narrowing (stenosis) of the renal arteries. This reduces blood flow to the kidneys and triggers the RAAS, resulting in elevated blood pressure.
Chronic hypertension can lead to hypertensive nephropathy, a type of CKD caused by sustained high blood pressure that damages the blood vessels and filtering units (glomeruli) in the kidneys. This damage impairs the kidneys' ability to effectively remove waste and excess fluids from the body, which can raise blood pressure even more (32). Hypertensive nephrosclerosis is the second leading cause of end-stage renal disease (ESRD).
The interplay between kidney health and high blood pressure is intrinsically bidirectional. This reciprocal relationship means that hypertension and kidney disease can progressively worsen each other, highlighting the importance of early detection and comprehensive management to break the cycle and preserve both cardiovascular and renal health.
Importance of Testing Kidney Function in Hypertensive Patients
According to one study, 11.4% of hypertensive patients exhibit abnormalities in renal function measurable through biochemical testing. Since kidney dysfunction can both cause and result from high blood pressure, renal function tests are essential in the initial diagnostic evaluation and ongoing management of hypertension.
Laboratory testing aids in diagnosing and classifying hypertension while also enabling the regular monitoring of kidney function in patients with high blood pressure. By tracking changes in renal health over time, healthcare providers can detect kidney dysfunction early, evaluate the effectiveness of hypertension management strategies, and make informed medical decisions to prevent the development and progression of CKD and ESRD.
Top Lab Tests for Monitoring Kidney Health in Hypertensive Patients
Kidney function is best assessed with routine renal function markers that collectively provide a comprehensive overview of how well the kidneys perform their various roles in the body.
Comprehensive Metabolic Panel (CMP)
The CMP is a broad screening tool that evaluates overall metabolic health, including kidney function. Regular CMPs help monitor the effects of hypertension and its treatments on renal and metabolic health.
Pertinent CMP Components
Glomerular filtration rate (GFR) measures the rate at which the kidneys filter blood. It is calculated using equations such as the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation, which accounts for age, sex, and serum creatinine levels. GFR categorizes kidney function and disease into stages:
Serum creatinine assesses the kidneys' ability to filter waste products from the blood. Normal serum creatinine levels vary by age, sex, and muscle mass, typically ranging from 0.6 to 1.4 mg/dL in adults. Elevated levels may indicate impaired kidney function, but factors such as muscle mass, diet, and certain medications can also influence results. (12)
Blood urea nitrogen (BUN) evaluates kidney function and protein metabolism by measuring the amount of nitrogen in the blood derived from urea. Normal BUN levels range from 6 to 20 mg/dL. Elevated BUN may suggest kidney dysfunction, dehydration, or high protein intake. (18)
Albumin is a water-soluble protein found in the blood. The normal range is typically between 3.5-5.5 g/dL (23). Low levels can indicate kidney disease, as damaged kidneys allow albumin to pass from the blood into urine. (2)
Electrolyte panels monitor key minerals affected by kidney function, including sodium, potassium, and calcium. Hyperkalemia (high potassium), hyponatremia (low sodium), and hypocalcemia (low calcium) are common consequences of kidney disease that can be detected on a CMP.
Dipstick Urinalysis
When the kidneys' glomeruli are damaged, they begin to leak specific substances into the urine that can be detected through urinalysis. Dipstick urinalysis is a quick, non-invasive, and inexpensive diagnostic test that uses a chemically treated strip to identify these substances. Indicators of kidney damage on a dipstick test include the presence of protein (proteinuria), blood (hematuria), or glucose (glucosuria) in the urine (45).
Urine Albumin-to-Creatinine Ratio (uACR)
uACR detects early kidney damage, even if GFR is normal, by measuring the amount of albumin excreted in the urine relative to creatinine. It is also used to stage CKD:
In addition to being an early marker of kidney disease, an elevated uACR also signifies an increased risk for cardiovascular events.
Cystatin C
Cystatin C is a protein filtered by the kidneys, serving as an alternative marker for estimating GFR. Compared to creatinine, cystatin C levels are less influenced by diet and muscle mass, providing a more accurate assessment of kidney function in certain populations. Normal ranges vary between 0.53 and 1.02 mg/L. High levels can indicate kidney dysfunction.
Hemoglobin A1c (HbA1c)
Diabetes is the leading cause of kidney failure in the United States. Over time, high blood sugar damages the small blood vessels in the kidneys and causes kidney scarring. These vascular changes can further contribute to high blood pressure. About one-third of people with diabetes develop kidney disease. (24)
HbA1c assesses long-term blood sugar control, reflecting average blood sugar over the last two to three months. It is the preferred biochemical marker for diagnosing and monitoring diabetes.
Normal HbA1c levels are below 5.7%. Levels between 5.7 and 6.4% indicate prediabetes. Levels of 6.5% or higher are diagnostic for diabetes. Optimal targets for diabetic patients typically aim for HbA1c below 7%. (13)
Lipid Profile
Dyslipidemia is associated with an increased risk of atherosclerosis and renal artery stenosis, contributing to hypertension and kidney damage.
Components of a lipid panel include:
- Total cholesterol
- Low-density lipoprotein cholesterol (LDL-C)
- Very low-density lipoprotein cholesterol (VLDL-C)
- High-density lipoprotein cholesterol (HDL-C)
- Triglycerides
According to the Physicians' Health Study, patients with the following cholesterol problems were more likely to develop kidney disease over time:
- Elevated total cholesterol
- High non-HDL cholesterol
- High total cholesterol/HDL ratio
- Low HDL-C
High-Sensitivity C-Reactive Protein (hs-CRP)
hs-CRP is an acute-phase protein that quantifies inflammation levels. Elevated hs-CRP has been associated with an increased risk of cardiovascular disease and all-cause mortality, and it has also been identified as a risk factor for acute kidney injury and CKD (42, 44).
Complete Blood Count (CBC)
Erythropoiesis is the process by which red blood cells (RBCs) are produced in the bone marrow. The kidneys produce erythropoietin (EPO), which stimulates the bone marrow to increase RBC production.
As kidney function declines, EPO synthesis decreases, leading to reduced stimulation of the bone marrow and, consequently, lower production of RBCs (4). This condition is known as anemia of chronic disease. Characteristics of this anemia include:
- Normocytic: RBCs are of normal size
- Normochromic: RBCs have a normal red color
A CBC measures various markers related to the number and qualities of red blood cells in circulation, assisting in diagnosing and managing anemia. CBC findings consistent with anemia of chronic disease include:
- Low RBC count, hemoglobin, and hematocrit
- Normal mean corpuscular volume, mean corpuscular hemoglobin, and mean corpuscular hemoglobin concentration
Vitamin D
The kidneys are also responsible for converting vitamin D to its active form (calcitriol). Vitamin D deficiency (defined as a serum calcidiol level below 30 ng/mL) is highly prevalent in CKD, affecting more than 80% of patients.
Vitamin D deficiency can increase the risk of hypertension and advanced-stage CKD. It can also worsen anemia of chronic disease.
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Key Takeaways
- Hypertension and kidney function are closely interconnected, with each condition significantly impacting the other. Chronic high blood pressure can damage the kidneys' blood vessels, impairing their ability to function. Compromised kidney function can exacerbate hypertension by disrupting the body's mechanisms for controlling blood pressure.
- Recognizing the critical interplay between these two health issues underscores the importance of regular kidney function monitoring in managing hypertension.
- By integrating routine renal function testing into the clinical management of hypertension, healthcare providers can detect early signs of kidney impairment, assess the effectiveness of hypertension treatments, and make informed decisions to adjust therapies as needed.
- This proactive approach enhances patient outcomes by preventing the progression of hypertension-related kidney disease and promotes overall cardiovascular health.
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