Biochemistry
Biochemical Laboratory Tests
Clinical biochemistry for MBBS: colorimetry and spectrophotometry, urine analysis, renal and liver function tests, lipid profile and cerebrospinal fluid, mapped to NMC codes BI11.1 to BI11.20.
MedNext Academy | 3 min read
Biochemical Laboratory Tests
Clinical biochemistry for MBBS: colorimetry and spectrophotometry, urine analysis, renal and liver function tests, lipid profile and cerebrospinal fluid, mapped to NMC codes BI11.1 to BI11.20.
This chapter covers the laboratory methods and clinical tests that underpin diagnostic biochemistry. It runs from colorimetry, spectrophotometry and buffer preparation to urine analysis, renal and liver function tests, the lipid profile and cardiac markers, and the interpretation of body fluids such as cerebrospinal fluid.
High-yield: Biochemical Laboratory Tests
- Colorimetry measures the concentration of a coloured solution, following the Beer-Lambert law that absorbance is proportional to concentration.
- Spectrophotometry extends this to specific wavelengths and underlies most quantitative biochemistry assays.
- A buffer is prepared from a weak acid and its salt, and its pH is set by the Henderson-Hasselbalch equation and measured with a pH meter.
- Normal urine contains water, urea, creatinine, uric acid, sodium, potassium and chloride, but no glucose, protein, ketones or blood.
- Glycosuria appears when blood glucose exceeds the renal threshold of about 180 mg/dL, as in uncontrolled diabetes.
- Proteinuria points to glomerular disease, and persistent microalbuminuria is an early sign of diabetic nephropathy.
- Ketonuria signals fasting, starvation or diabetic ketoacidosis.
- Serum creatinine and creatinine clearance estimate the glomerular filtration rate and renal function.
- The albumin to globulin ratio is normally about 1.2 to 2.0; it reverses in chronic liver disease and multiple myeloma.
- A lipid profile of total cholesterol, HDL, LDL and triglycerides assesses cardiovascular risk.
- Raised aspartate and alanine aminotransferases indicate hepatocellular damage, and alkaline phosphatase rises in cholestasis and bone disease.
- Serum bilirubin is fractionated into conjugated and unconjugated to separate prehepatic, hepatic and posthepatic jaundice.
- Cardiac markers such as troponins and creatine kinase-MB support the diagnosis of myocardial infarction.
- Cerebrospinal fluid is normally clear with glucose about two-thirds of blood glucose and low protein; it is measured to diagnose meningitis.
- Paper chromatography of urine amino acids screens for inborn errors of metabolism such as aminoacidurias.
Core biochemical investigations
- **Renal function:** Serum creatinine, urea and creatinine clearance estimate glomerular filtration rate.
- **Liver function:** Aminotransferases for hepatocellular injury, alkaline phosphatase and bilirubin for cholestasis.
- **Lipid profile:** Total cholesterol, HDL, LDL and triglycerides for cardiovascular risk.
- **Urine analysis:** Glucose, protein, ketones and blood flag diabetes, renal and metabolic disease.
NMC competencies in this chapter
- **BI11.1:** Laboratory apparatus, safe practice and waste disposal
- **BI11.3:** Chemical components of normal urine
- **BI11.6:** Principles of colorimetry
- **BI11.7:** Estimation of serum creatinine and creatinine clearance
- **BI11.15:** Composition of cerebrospinal fluid
- **BI11.17:** Basis of biochemical tests in diabetes, dyslipidaemia and myocardial infarction
- **BI11.18:** Principles of spectrophotometry
- **BI11.20:** Interpretation of abnormal urine constituents and their correlation with disease
Frequently Asked Questions
What principle underlies colorimetry?
The Beer-Lambert law, which states that the absorbance of light by a coloured solution is directly proportional to the concentration of the absorbing substance and the path length.
Why do we fractionate serum bilirubin?
Separating conjugated from unconjugated bilirubin distinguishes the causes of jaundice. Unconjugated rises in haemolysis, while conjugated rises in obstruction and many hepatic conditions.
What does the albumin to globulin ratio tell us?
A normal ratio is roughly 1.2 to 2.0. It falls or reverses in chronic liver disease, where albumin drops, and in multiple myeloma, where globulins rise.
How does cerebrospinal fluid analysis help in meningitis?
The pattern of glucose, protein and cell count separates the causes. Bacterial meningitis lowers glucose and raises protein and neutrophils, whereas viral meningitis often keeps glucose normal.
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