Kidneys

Fasting Glucose measures blood sugar levels after an overnight fast (typically 8-12 hours). It reflects baseline glucose regulation. While standard guidelines define diabetes risk based on thresholds (e.g., prediabetes 100-125 mg/dL, diabetes ≥126 mg/dL), longevity experts emphasize maintaining levels in a tighter, lower optimal range (e.g., 70-90 mg/dL) for better metabolic health, reduced glycation, and potentially lower risk of age-related diseases. Continuous Glucose Monitoring (CGM) provides much richer data on glucose variability and post-meal responses.

Blood Urea Nitrogen (BUN) measures the amount of nitrogen in the blood that comes from urea, a waste product formed in the liver from protein metabolism and excreted by the kidneys. Elevated BUN can indicate decreased kidney function, dehydration, high protein intake, or gastrointestinal bleeding. Low BUN is less common but can occur with liver disease or malnutrition. Often interpreted alongside creatinine (BUN/Creatinine ratio).

Serum Creatinine is a waste product generated from muscle metabolism (creatine breakdown). It is filtered out of the blood by the kidneys, so blood levels are a key indicator of kidney filtration function when considered alongside factors like age, sex, race, and muscle mass. Higher levels generally indicate poorer kidney function. Use eGFR (Estimated Glomerular Filtration Rate) calculated from Creatinine for a better assessment of kidney function.

Estimated Glomerular Filtration Rate (eGFR, creatinine-based) is the standard measure of kidney function, estimating how well the kidneys are filtering waste from the blood. It is calculated using formulas (CKD-EPI 2021) that incorporate serum creatinine, age, and sex. Creatinine comes from muscle, so this estimate can be less precise in people with very low or very high muscle mass. Lower eGFR indicates poorer kidney function. Chronic Kidney Disease (CKD) stages are defined by eGFR levels (e.g., Stage 3 <60 mL/min/1.73m²). Maintaining eGFR ≥90 is generally considered ideal; a persistent decline below 60 suggests significant impairment and should be evaluated by a clinician.

BUN/Creatinine Ratio compares blood urea nitrogen to creatinine levels. An elevated ratio (>20:1) often suggests pre-renal causes of kidney injury (like dehydration or decreased blood flow to kidneys), GI bleeding, or high protein intake, as BUN rises disproportionately to creatinine. A low ratio (<10:1) might indicate liver disease (low urea production) or low protein intake.

Serum Sodium is the main cation in extracellular fluid and crucial for maintaining fluid balance, nerve impulse transmission, and muscle function. Levels are tightly regulated by the kidneys and hormones. Abnormal levels (hyponatremia or hypernatremia) usually reflect disturbances in water balance rather than sodium intake, often related to kidney function, hormonal issues (like ADH), or excessive fluid loss/gain.

Serum Potassium is the primary intracellular cation, essential for nerve conduction, muscle contraction (especially the heart), and fluid balance. Serum levels are kept within a narrow range, primarily regulated by the kidneys. Abnormal levels (hypokalemia or hyperkalemia) can cause serious cardiac arrhythmias and muscle weakness, resulting from kidney disease, medications (diuretics, ACE inhibitors), hormonal issues, or severe fluid loss.

Serum Chloride is the major anion in extracellular fluid, working with sodium to maintain fluid balance and acid-base status. Levels often mirror sodium levels. Abnormal chloride levels can result from dehydration, kidney disease, excessive vomiting or diarrhea, or acid-base disturbances (like metabolic acidosis or alkalosis).

Total Carbon Dioxide (CO2) content in serum primarily reflects the level of bicarbonate (HCO3-), a major component of the body's acid-base buffering system managed by the kidneys and lungs. Abnormal levels indicate acid-base imbalances: low levels suggest metabolic acidosis or respiratory alkalosis, while high levels suggest metabolic alkalosis or respiratory acidosis.

Serum Calcium measures the total amount of calcium (bound to albumin and free/ionized) in the blood. Calcium is essential for bone health, muscle contraction, nerve signaling, and blood clotting. Levels are tightly regulated by parathyroid hormone (PTH) and vitamin D. Abnormal levels can indicate issues with the parathyroid glands, vitamin D status, kidney function, or certain cancers. Ionized calcium is the biologically active form and may be more accurate if albumin is abnormal.

Serum Phosphorus (Phosphate) measures inorganic phosphate levels in the blood. Phosphorus is essential for bone structure (hydroxyapatite), energy transfer (ATP), DNA/RNA structure, and cell membranes. Levels are regulated by the kidneys, PTH, and vitamin D, often in inverse relationship with calcium. High levels (hyperphosphatemia) are common in kidney failure or with excessive intake/low PTH. Low levels (hypophosphatemia) can occur with malnutrition, refeeding syndrome, high PTH, or vitamin D deficiency.

Serum Albumin is the most abundant protein in the blood, synthesized by the liver. It maintains oncotic pressure (keeping fluid in blood vessels), transports hormones, fatty acids, and drugs, and acts as an antioxidant and anti-inflammatory mediator. Low levels can indicate liver disease, kidney disease (loss in urine), malnutrition, chronic inflammation, or infection (negative acute phase reactant). High levels usually suggest dehydration.

Cystatin C is a protein produced by most cells in the body and filtered by the kidneys at a constant rate. Unlike creatinine, its levels are less affected by muscle mass or diet, making it potentially a more sensitive marker of early kidney dysfunction, especially when creatinine-based eGFR is borderline or muscle mass is atypical. Elevated levels indicate reduced glomerular filtration rate. Use eGFR calculated from Cystatin C (eGFRcys) or combined Creatinine-Cystatin C (eGFRcr-cys) for the best assessment of kidney function.

Estimated Glomerular Filtration Rate (eGFR, cystatin-C-based) is a measure of kidney function that uses cystatin C—a protein produced at a relatively constant rate by all nucleated cells—along with age and sex. Unlike creatinine, cystatin C is not influenced by muscle mass, making it especially useful in people with unusually high or low muscle mass, the elderly, or those with borderline creatinine-based eGFR values. It is calculated using the CKD-EPI 2012 cystatin C equation. eGFR is reported on the same scale (mL/min/1.73m²), and CKD stages use the same cutoffs (e.g., Stage 3 <60). Values ≥90 are generally ideal, while sustained values below 60 indicate reduced kidney function and warrant medical follow-up.

Measures creatinine concentration in random urine. Used to normalize other urine measurements and assess kidney function.

Detects albumin, a protein produced by the liver. Elevated levels can indicate kidney disease, liver disease, or certain medications.

Ratio of albumin to creatinine in random urine. Used to screen for and monitor kidney disease, especially in diabetes.