Overview
Clinical chemistry measures chemical changes in body fluids — serum, plasma, and urine — to support the diagnosis, therapy, and prognosis of disease. It is one of the busiest sections of any medical laboratory, covering everyday tests such as blood glucose, urea, calcium, lipid profile, and urinalysis chemistry.
This lesson brings together the regulation of blood sugar and its laboratory tests, blood urea nitrogen (BUN) as a marker of renal function, calcium-phosphate homeostasis, the lipid profile used in cardiovascular risk assessment, and the interpretation of glucose, creatinine, and protein in urine.
Learning Objectives
After this lesson you will be able to…- Describe the regulation of blood sugar and identify the major blood glucose laboratory tests
- Explain calcium and phosphate homeostasis and their normal reference ranges
- Describe the components and clinical use of the lipid profile
- Interpret urine creatinine, glycosuria, and proteinuria findings
- Correlate abnormal clinical chemistry results with underlying disease processes
Clinical Story
Why This MattersA 45-year-old woman with a family history of diabetes comes for a routine health check. Her fasting blood glucose returns at 138 mg/dL — above the normal reference range. The physician also orders a lipid profile and urine analysis. The laboratory technologist's understanding of glucose regulation, testing methodology, and reference ranges is essential to flag this abnormal result appropriately and support the physician's diagnostic workup for diabetes mellitus.
Core Concepts
Blood glucose is tightly regulated by two antagonistic hormone systems: catabolic hormones (glucagon, cortisol, catecholamines) raise glucose, while the anabolic hormone insulin lowers it. Normal fasting glucose is 70–100 mg/dL. Persistently high levels (hyperglycemia) most often indicate diabetes mellitus; low levels (hypoglycemia) can be life-threatening. Key laboratory tests include: fasting blood sugar (FBS), 2-hour postprandial blood sugar (2-h PPBS), oral glucose tolerance test (OGTT), intravenous glucose tolerance test (IVGTT), glycosylated haemoglobin (HbA1c), and random blood sugar (RBS). The two most commonly used enzymes for glucose measurement are glucose oxidase and hexokinase.
Urea is produced by the liver in the urea cycle as a waste product of protein digestion. Normal blood urea nitrogen is 6–20 mg/dL (8–20 mmol/L). BUN rises when the glomerular filtration rate (GFR) and blood volume decrease, and is also elevated by fever, high protein diet, increased catabolism, and gastrointestinal bleeding — making it a key indicator of renal health.
Calcium is the most abundant mineral in the body (≈1 kg, 99% in the skeleton). Normal total serum calcium is 2.2–2.6 mmol/L (9–10.5 mg/dL); ionized calcium is 1.1–1.4 mmol/L. Calcium has structural (bone), signalling (second messenger), and enzymatic (clotting cofactor) roles. Absorption depends on vitamin D and calbindin; excretion and reabsorption are regulated by parathyroid hormone (PTH) and calcitonin.
The lipid profile is a broad screening panel for cardiovascular risk, including LDL (60–130 mg/dL, 'bad' cholesterol), HDL (>40 mg/dL, 'good' cholesterol), triglycerides (10–150 mg/dL), and total cholesterol (<200 mg/dL). Traditionally patients fast 9–12 hours before testing, though many laboratories now accept non-fasting samples. Elevated values indicate hyperlipidemia, a recognised risk factor for cardiovascular disease and pancreatitis.
Glycosuria occurs when blood glucose exceeds the renal threshold (~160–180 mg/dL), overwhelming tubular reabsorption; it is identified using Benedict's qualitative test. Creatinine, a muscle metabolism by-product, is used to estimate GFR; normal serum values are 0.5–1.0 mg/dL (women) and 0.7–1.2 mg/dL (men). Proteinuria (>150 mg/day) signals glomerular disease, overflow states, or tubular reabsorption defects (Fanconi syndrome), and is often expressed as a protein/creatinine ratio (PCR).
Laboratory Principle
Most clinical chemistry glucose assays use enzyme-coupled colorimetric or amperometric reactions: glucose oxidase or hexokinase specifically converts glucose to a measurable product, generating a colour change or electrical current proportional to glucose concentration. Similarly, calcium, urea, and lipid assays rely on specific enzymatic or chemical reactions that produce a coloured end-product measured spectrophotometrically, allowing precise, automatable quantification.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| Glucose oxidase-peroxidase (GOD-POD) reagent | Enzymatic, working strength | Quantifies blood glucose colorimetrically | 2–8°C |
| Urease/GLDH reagent | Enzymatic, working strength | Measures blood urea nitrogen | 2–8°C |
| O-cresolphthalein complexone reagent | Working strength | Colorimetric total calcium determination | 2–8°C, protect from light |
| Cholesterol esterase/oxidase reagent | Enzymatic, working strength | Total cholesterol and HDL/LDL measurement | 2–8°C |
| Fluoride oxalate tubes | Sodium fluoride 2 mg/mL blood | Prevents in-vitro glycolysis for glucose transport/storage | Room temperature |
Step-by-Step Procedure
Confirm fasting status (8–12 hours) for glucose, lipid profile, and calcium as required by the requested test.
Draw venous blood into the appropriate tube (fluoride oxalate for glucose transport, plain/serum separator for other chemistries).
Centrifuge at 3,000 rpm for 10 minutes; separate serum/plasma promptly to prevent glycolysis and analyte degradation.
Load samples onto the automated analyzer; the instrument adds reagent, incubates at 37°C, and measures the colorimetric/enzymatic reaction.
Check quality control results, verify against reference ranges and delta checks, then release the validated report to the physician.
Flow Diagram
Quality Control
Run two levels of commercial control material (normal and abnormal) with each analytical run for glucose, urea, calcium, and lipid tests. Track results on Levey-Jennings charts and apply Westgard multi-rule QC before releasing patient data.
Enroll in an accredited External Quality Assessment Scheme (e.g. national clinical chemistry proficiency testing) to benchmark glucose, lipid, and renal chemistry performance against peer laboratories quarterly.
Reference Values
Normal Ranges⚠️ Reference ranges may vary between laboratories. Always apply your laboratory's established reference intervals.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Fasting glucose > 125 mg/dL | Suggestive of diabetes mellitus | Confirm with repeat FBS, OGTT, or HbA1c |
| LDL elevated, HDL low | Increased cardiovascular risk (dyslipidemia) | Lifestyle counselling; consider lipid-lowering therapy |
| BUN elevated with normal creatinine | Possible pre-renal cause: dehydration, GI bleed, high protein diet | Assess hydration status and dietary history |
| Glucose in urine (glycosuria) with normal blood glucose | Possible renal glycosuria (low renal threshold) | Confirm with fasting blood glucose; refer if isolated finding |
Common Errors & How to Avoid Them
Cause: Ongoing glycolysis by red/white cells falsely lowers glucose if analysis is delayed
Prevention: Use fluoride-oxalate tubes when testing will be delayed, and separate promptly.
Cause: Recent food intake raises triglycerides and can affect calculated LDL
Prevention: Verify fasting status before collection; reschedule if not fasted.
Cause: Interferes with colorimetric reaction, causing inaccurate results
Prevention: Reject grossly haemolysed/lipemic samples and redraw.
Laboratory Tips from the Bench
Whole blood glucose reads lower than plasma/serum glucose because red cells contain less water per unit volume — always confirm which sample type your reference range applies to.
The biologic effect of calcium depends on the ionized fraction, not total calcium — always check serum albumin when calcium results appear discordant with symptoms.
Remember lipid profile letters as 'Lousy LDL, Healthy HDL' — LDL is the 'bad' cholesterol that deposits in arteries, HDL is the 'good' cholesterol that clears it.
Important Notes
Glycosuria typically appears only once blood glucose exceeds ~160–180 mg/dL (the renal threshold); children and pregnant women may have a lower threshold, causing glycosuria even at normal blood glucose.
While traditional lipid testing required 9–12 hours of fasting, many modern guidelines now accept non-fasting lipid panels for general screening — check your laboratory's current protocol.
Interactive Quiz
Test Your KnowledgeFlashcards
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Clinical Case Study
Apply Your KnowledgeAttends a routine annual health check with no acute complaints but a strong family history of type 2 diabetes. She reports increased thirst and fatigue over the past two months.
A fasting glucose above 125 mg/dL together with an elevated HbA1c confirms a diagnosis of diabetes mellitus. Her lipid profile shows elevated LDL and low HDL, consistent with diabetic dyslipidemia, which further raises her cardiovascular risk.
- →Fasting blood glucose ≥126 mg/dL on two occasions or HbA1c ≥6.5% confirms diabetes mellitus.
- →Diabetic patients commonly present with dyslipidemia — elevated LDL/triglycerides and low HDL.
- →Laboratory results must always be interpreted alongside clinical history and repeated for confirmation.
Frequently Asked Questions
Fasting eliminates the transient rise in glucose after meals, giving a stable baseline that reflects the body's true glucose homeostasis rather than a temporary postprandial spike.
BUN is affected by protein intake, hydration, and catabolism, making it less specific. Creatinine, a constant by-product of muscle metabolism, more reliably reflects glomerular filtration rate, though both are used together (BUN:creatinine ratio) for a fuller picture.
HDL particles transport excess cholesterol from peripheral tissues (including artery walls) back to the liver for excretion — a process called reverse cholesterol transport — reducing the risk of atherosclerotic plaque formation.
Quick Revision
10-Minute ReviewKey Takeaways
- Blood glucose is tightly regulated by insulin (lowers) versus glucagon, cortisol, and catecholamines (raise).
- Key glucose tests include FBS, 2-h PPBS, OGTT, IVGTT, HbA1c, and random blood sugar.
- BUN and creatinine together assess renal function; creatinine is the more reliable GFR marker.
- The lipid profile (LDL, HDL, triglycerides, total cholesterol) guides cardiovascular risk assessment.
- Urine glucose, creatinine, and protein tests provide critical insight into renal and metabolic health.
Competency Checklist
Track Your MasteryReferences
- National Institute of Open Schooling. Biochemistry — Module: Clinical Chemistry (Lesson 7).
- Burtis CA, Ashwood ER, Bruns DE. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 5th ed. Elsevier.
- American Diabetes Association. Standards of Medical Care in Diabetes.