ORGAN FUNCTION TESTS: A TEACHER’S COMPREHENSIVE GUIDE TO KIDNEY AND LIVER FUNCTION TESTS
Welcome, future pharmacists and healthcare professionals!
Organ function tests are essential diagnostic tools used to assess the health and functioning of various organs in the body. These tests help in early detection, diagnosis, and monitoring of diseases. In this comprehensive guide, we will explore the functions of kidneys and liver, along with the various tests used to evaluate their function and their clinical significance.
Dpharmguru’s exam insights:
In my years of teaching clinical biochemistry, I have observed that students often get confused between different kidney function tests. Remember: BUN and creatinine are the two most important tests for kidney function. BUN measures urea nitrogen, while creatinine is a more specific indicator of kidney function. This is a common exam question!
PART 1: KIDNEYS
Kidneys are present in pair in vertebrates as well as in some invertebrates. They are the major organs of the urinary system. They produce urine through which waste materials such as urea and ammonia are excreted. They also reabsorb glucose and amino acids. Along with this, kidneys also perform secretory function, e.g., calcitriol, erythropoietin, and renin are some of the hormones produced by kidneys.
Functions of Kidney
- Regulation of Blood Ionic Composition: They regulate the concentration of some ions (Na⁺, K⁺, Ca²⁺, Cl⁻, and HPO₄²⁻) in blood.
- Regulation of Blood pH: They regulate blood pH by excreting H⁺ ions into the urine and conserving HCO₃⁻ ions.
- Regulation of Blood Volume: They regulate blood volume by conserving water or eliminating the excess in urine.
- Regulation of Blood Pressure: They regulate blood pressure by secreting renin enzyme which activates the renin-angiotensin-aldosterone pathway.
- Maintenance of Blood Osmolarity: They maintain a constant blood osmolarity close to 300 milliosmoles per litre (mOsm/litre).
- Production of Hormones: They produce calcitriol (active form of vitamin D) and erythropoietin (stimulates RBC production).
- Regulation of Blood Glucose Level: They utilise glutamine amino acid in gluconeogenesis.
- Excretion of Wastes and Foreign Substances: They form urine to excrete waste materials like ammonia, urea, bilirubin, creatinine, and uric acid.
Kidney Function Tests and their Clinical Significance
1. Blood Urea Nitrogen Test (BUN)
Urea (by-product of protein metabolism) is a waste product that is produced in the liver, filtered from the blood, and then excreted in the urine by the kidneys. The BUN test evaluates the nitrogen content in urea. High levels of BUN indicate kidney dysfunction. Protein intake and liver function also affect BUN, and thus this test is performed in combination with a blood creatinine test.
2. Serum Creatinine Test
This test evaluates the blood level of creatinine (a by-product of muscle energy metabolism), which is filtered from the blood and excreted in the urine by the kidneys. In normal kidney function, the blood level of creatinine remains comparatively constant. Since creatinine is slightly affected by liver function, an increased blood creatinine is a highly sensitive indication of impaired kidney function than the BUN.
Dpharmguru’s exam insights:
A common exam question is: “Which is a more sensitive indicator of kidney function – BUN or Serum Creatinine?” The answer is Serum Creatinine because it is less affected by liver function and protein intake. Remember: Creatinine is the gold standard for kidney function assessment!
3. Uric Acid Test
Uric acid is the end product of purine metabolism. Hyperuricaemia is the result of overproduction of uric acid (increased destruction of nucleoproteins, high protein diet, or inborn enzymatic flaws). Asymptomatic hyperuricaemia is categorised as an increased serum uric acid without signs of acute gouty arthritis. When serum urate concentration exceeds the saturation point (420 µmol/L or 7 mg/dL), crystals of monosodium urate accumulate in and around the joints, cartilage, and in kidneys, causing gout.
4. Other Blood Tests
Evaluation of the blood levels of other elements partially regulated by kidneys, e.g., sodium, potassium, chloride, bicarbonate, calcium, magnesium, phosphorus, protein, uric acid, and glucose, can be helpful in estimating the function of kidney.
Normal Reference Values
- Creatinine Clearance: Normal results for a 24-hour urine collection are 90-139 ml/min for adult males and 80-125 ml/min for adult females below 40 years old.
- Urea Clearance: The normal range is 64-99 ml/min with maximum clearance.
- Urine Osmolality: With limited fluid intake, osmolality should be more than 800 mOsm/kg of water. With increased fluid intake, osmolality should be below 100 mOsm/kg.
- Urine Protein: A 24-hour urine collection should contain not more than 150 mg of protein.
- Blood Urea Nitrogen (BUN): The normal range is 8-20 mg/dL.
- Creatinine: The normal range for males is 0.8-1.2 mg/dL and for females is 0.6-0.9 mg/dL.
PART 2: LIVER
Liver is the largest gland of the human body. It forms the second largest organ (skin is the largest organ). It lies just inferior to the diaphragm in the right side of the abdomino-thoracic region. It weighs about 1.4 kg, is reddish-brown in colour, and feels rubbery when touched.
Functions of Liver
- Carbohydrate Metabolism: Plays a significant role in maintenance of normal blood glucose level. Glycogen can be broken down into glucose and released into the blood by the liver.
- Lipid Metabolism: Liver cells store triglycerides; breakdown fatty acids to produce ATP; synthesise lipoproteins; synthesise cholesterol; and produce bile salts.
- Protein Metabolism: Deamination of amino acids takes place in the liver. Most of the plasma proteins like albumin, prothrombin, and fibrinogen are synthesised by hepatocytes.
- Processing of Drugs and Hormones: Detoxification of substances like alcohol occurs in the liver. Excretion of drugs like penicillin, erythromycin, and sulfonamides occurs into bile.
- Excretion of Bilirubin: The heme of aged RBCs produces bilirubin which is absorbed by the liver from the blood and is secreted into bile.
- Synthesis of Bile Salts: Emulsification and absorption of lipids in the small intestine utilises the bile salts.
- Storage: Liver forms a primary site for the storage of certain vitamins (A, B₁₂, D, E, and K) and minerals (iron and copper) along with glycogen.
- Phagocytosis: The aged RBCs, WBCs, and some bacteria undergo phagocytosis by the Kupffer cells of the liver.
- Activation of Vitamin D: The active form of vitamin D is synthesised by the skin, liver, and kidneys.
Liver Function Tests and their Clinical Significance
1. Serum Bilirubin
It is the breakdown product of haemoglobin and is the major pigment in bile.
Normal Range: Total serum bilirubin is 0.3-1.3 mg/dL and direct bilirubin is 0.1-0.4 mg/dL.
Clinical Significance:
- Increased level of bilirubin indicates jaundice.
- Increased level of total bilirubin indicates haemolysis.
- Increased level of total and direct bilirubin indicates intrahepatic or extra-hepatic biliary obstruction.
- Increased level of indirect bilirubin indicates intrahepatic cholestasis.
- Increased level of direct and indirect bilirubin indicates liver cell necrosis.
2. Serum (Plasma) Proteins
Albumin and globulins (α, β, and γ) are the prime plasma proteins.
Normal Range: Total serum proteins is 6-8 g/dL; serum albumin is 3.5-5.1 g/dL; serum globulin is 1.8-3.1 g/dL; albumin to globulin ratio lies between 0.8 to 2.0.
Dpharmguru’s exam insights:
A common exam question is: “What is the normal A:G ratio?” The answer is 0.8 to 2.0. Also, remember that decreased albumin indicates liver disease (since albumin is synthesised in the liver), while increased globulin indicates chronic infections.
3. Alkaline Phosphatase (ALP)
This enzyme is produced in the liver, bones, small intestine, and kidneys. It acts as a catalyst which facilitates the splitting of phosphate group from monophosphoric esters.
Normal Range: 29-92 IU/L.
Clinical Significance: Low levels of ALP indicate hypophosphatasia; while high levels of ALP indicate rickets, osteomalacia, obstructive jaundice, and impaired absorption of vitamin D and calcium.
4. SGOT (AST) and SGPT (ALT)
SGOT (AST): This enzyme is mainly found in the heart and liver, and to a lesser extent in skeletal muscles, kidneys, and pancreas. Normal Range: 0-40 U/L. Increased levels indicate liver diseases, cardiac diseases, and skeletal muscle damage.
SGPT (ALT): This enzyme is mainly found in the liver cells. Normal Range: 5-36 U/L. Increased levels indicate liver cell damage.
Dpharmguru’s exam insights:
The difference between AST (SGOT) and ALT (SGPT) is frequently tested. AST is found in the heart, liver, and muscles, while ALT is more specific to the liver. An elevated ALT is a more specific indicator of liver damage than AST. This is a very common exam question!
5. Serum Cholesterol (Lipid Profile)
This blood test calculates the blood’s triglyceride and cholesterol levels.
Lipid Profile Parameters
- Total Cholesterol Level: Total amount of cholesterol in blood. Normal: <200 mg/dL.
- LDL (Bad Cholesterol): Can accumulate in arteries. Normal: <100 mg/dL.
- HDL (Good Cholesterol): Helps keep LDL cholesterol out of arteries. Normal: ≥60 mg/dL.
- Triglycerides: Circulatory fats that provide energy. Normal: <150 mg/dL.
- VLDL Cholesterol: Derived from recently consumed food. Indicates faulty lipid metabolism if elevated.
Clinical Significance of Lipid Profile
- To identify whether cholesterol level is normal, borderline, moderate, or high-risk.
- To monitor cholesterol levels in patients with heart disease risk factors.
- To track the body’s response to cholesterol-lowering therapy.
- To assist in the diagnosis of liver disease.
SUMMARY
Organ function tests are essential diagnostic tools for assessing kidney and liver health. Kidney function tests include BUN, serum creatinine, uric acid, and electrolyte measurements. Liver function tests include serum bilirubin, serum proteins, ALP, AST, ALT, and lipid profile. Understanding these tests and their clinical significance is essential for pharmacy students and healthcare professionals to interpret laboratory results and make informed clinical decisions.
Dpharmguru’s exam insights:
Organ function tests are frequently tested in exams. Remember: (1) Creatinine is the most specific indicator of kidney function, (2) ALT is more specific for liver damage than AST, (3) ALP increases in biliary obstruction, and (4) Lipid profile assesses cardiovascular risk. These are almost guaranteed to appear in exams!
REFERENCES AND FURTHER READING
- Guyton, A. C., & Hall, J. E. (2020). Guyton and Hall Textbook of Medical Physiology (14th ed.). Elsevier.
- Park, K. (2022). Park’s Textbook of Preventive and Social Medicine (26th ed.). Banarsidas Bhanot Publishers.
- Murray, R. K., Bender, D. A., Botham, K. M., et al. (2018). Harper’s Illustrated Biochemistry (31st ed.). McGraw-Hill.
- National Institutes of Health (NIH). (2022). Kidney and Liver Function Tests Resources. Retrieved from https://www.nih.gov.
- World Health Organization (WHO). (2022). Laboratory Tests and Clinical Significance. Retrieved from https://www.who.int.
Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult qualified healthcare professionals for medical concerns.
written by:
Dr. N. Sujith Kumar
