Diagnostic Blood Tests for Kidney Function and Disease

1. Blood Nitrogen Waste Tests:

• Blood Urea Nitrogen (BUN): Measures nitrogenous waste (urea) in the blood.
• Creatinine: A waste product from muscle metabolism.
  • Elevated levels indicate kidney dysfunction because the kidneys fail to excrete these wastes effectively.

• Glomerular Filtration Rate (GFR):

  • defined as: the volume of blood plasma filtered by nephrons/minute.
  • Measuring GFR is the gold-standard for assessing Kidney Function. GFR should be 60 or higher.
  • Assessed by comparing serum levels of urea and creatinine with their urine concentrations over time.
  • A decreased GFR signifies impaired kidney filtration (glomeruli and/or nephron are impaired).
  • GFR of 15 or lower indicates kidney failure.

2. Blood pH and Acid-Base Balance:

• Metabolic Acidosis:
  • Occurs when kidneys fail to excrete excess hydrogen ions.
  • Results in increased acidity of the blood (pH <7.35).
  • Daily metabolism (enzymatic reactions) of the body typically produces acids that need to be excreted). The more acid, the more free H+ ions exist in the blood (typically in the form of carbonic acid and lactate).  Kidneys maintain blood pH7.4, by ensuring excess H+ ions are excreted in the form of urine.

3. Anemia:

• Caused by decreased secretion of erythropoietin (EPO):
  • Kidneys produce EPO, which stimulates red blood cell production.
  • Damage to kidneys leads to reduced EPO, causing less RBC formation.
  • Results in anemia, with decreased oxygen-carrying capacity.

4. Immune and Post-infectious Testing:

• Antibody detection:
  • Anti-streptolysin O and anti-streptokinase antibodies indicate recent or ongoing post-streptococcal glomerulonephritis (PSGN).
  • These antibodies were developed to eliminate Group A beta-hemolytic streptococcal infection and unfortunately cross-react with tissues of the body.  These antibodies may cause glomerular damage and inflammation, potentially leading to permanent kidney damage if untreated.

5. Microbial Cultures:

• Bacterial or viral cultures:  Used when infection is suspected.
  • Identify specific pathogens to guide appropriate antibiotic or antiviral therapy.

6. Renin Levels:

• Elevated serum renin indicates poor blood flow to the kidney (due to heart disease or other causes of shock).  The kidney is very susceptible to ischemia when blood flow is poor as there is no anastomoses (alternative blood vessels), and cells are not able to store oxygen.
• Cause:
  • Reduced blood flow to the kidney (e.g., artery occlusion, trauma), triggering excessive renin secretion.
  • Renin activates angiotensin II, causing vasoconstriction and stimulating aldosterone and ADH to increase blood volume and pressure (which unfortunately makes hypertension even worse).
• In cases of Hypertension, elevated serum renin suggests the kidney is the primary source of hypertension.
  • Indicates renal origin of hypertension—a potential target for therapy.
• Low levels of renin will occur during kidney failure when kidney cells are no longer able to produce renin.

Summary:

Kidney function tests include serum BUN, creatinine levels, and GFR to evaluate filtration efficiency. Elevated waste products, acid-base disturbances, and anemia suggest kidney damage. Serological tests for post-streptococcal infections and renin levels help diagnose specific causes of kidney-related hypertension or damage. Cultures and imaging aid in identifying infectious or structural pathologies, guiding targeted treatment.