Introduction to Clinical Biochemistry

Definition: Clinical biochemistry (also known as clinical chemistry or chemical pathology) is a branch of laboratory medicine in which chemical and biochemical methods are applied to the study of disease.
Clinical Significance: This laboratory service is absolutely essential for modern medical practice. It deals with laboratory applications used to discover the cause of a disease.
Clinical Outcomes: * Diagnosis: Biochemical investigations help clinicians determine the specific disease a patient is suffering from. * Prognosis: These investigations allow for the follow-up of treatment and the monitoring of recovery from illness.

The Use and Scope of Biochemical Tests

Branches of Medicine: Biochemical investigations are involved in every branch of clinical medicine.
Primary Applications: * Diagnosis and monitoring of treatment. * Screening for disease or assessing the prognosis. * Research into the biochemical basis of various diseases. * Clinical trials for evaluating new drugs.
Key Disease Indicators: Biochemical investigations are essential for the diagnosis and prognosis of: * Diabetes mellitus * Jaundice * Myocardial infarction * Gout * Pancreatitis * Rickets * Cancers * Acid-base imbalance

Screening and Unexpected Abnormal Test Results

Common Clinical Findings: * Hyperparathyroidism: Indicated by raised plasma calcium. * Hypothyroidism: Indicated by raised plasma TSH and/or a low $T_4$ . * Diabetes mellitus: Indicated by high random plasma glucose. * Renal tract disease: Indicated by raised plasma creatinine or urea. * Liver disease: Indicated by increased plasma ALT ( $\text{Alanine aminotransferase}$ ) and AST ( $\text{Aspartate aminotransferase}$ ).
Advantages of Screening: * Detection of uncommon or unexpected diseases. * Early requesting of a battery of tests might expedite the management of the patient. * Note: Most studies have not definitively shown that screening always expedites management.

Categories of Clinical Biochemical Tests

Clinical biochemical tests comprise over $1/3$ of all hospital laboratory investigations.
Core Biochemistry: Common tests provided by most laboratories on a frequent basis for many patients. These include: * Electrolytes: Sodium, potassium, chloride, and bicarbonate. * Renal Function: Urea and creatinine. * Minerals: Calcium and phosphate. * Proteins: Total protein and albumin. * Liver Function: Bilirubin and alkaline phosphatase (ALP). * Enzymes: Alanine aminotransferase (ALT), Aspartate aminotransferase (AST), and Amylase. * Metabolic: Glucose.
Specialized Tests: Performed in larger departments or reference centers that are equipped for less common requests: * Hormones * Specific proteins * Trace elements * Vitamins * Drugs * Lipids and lipoproteins * DNA analyses

Units, Reference Ranges, and Interpretation

Measurement Units: * Metabolites (e.g., glucose, urea): Expressed as $mg/dL$ or $mmol/L$ . * Electrolytes (e.g., $Na^+$ , $K^+$ ): Expressed as $mmol/L$ or $meq/L$ (older terminology). * Enzymes: Expressed as $Units/L$ .
Interpretation of Results: Values are interpreted as increased, decreased, or within the normal/reference range.
Reference Values: Obtained from healthy individuals under standardized statistical and laboratory conditions.
Normal (Reference) Range: The range within which $95\%$ of normal healthy persons fall. The cut-off values are calculated as: * $\text{Mean reference value} \pm N \times \text{Standard Deviation}$ * Where $N$ typically varies between $1$ , $2$ , or $3$ .

Common Laboratory Errors and Consequences

Crossover of Addressograph Labels: Can lead to two patients receiving each other's results. If a patient is assigned a completely wrong set of results, a full investigation is required to identify the second affected patient.
Timing Errors: Crucial for therapeutic drug monitoring and hormonal tests. Sending blood too early after drug administration results in misleadingly high values. Cortisol interpretation depends critically on the time of day the sample was taken.
Sample Collection Tube Error: * Using lithium-heparin tubes for therapeutic lithium level measurement invalidates the test. * Serum electrophoresis requires serum; fibrinogen in plasma interferes with monoclonal band detection. * "Topping up" a biochemistry tube with a hematology sample (Potassium-EDTA) leads to high potassium and low calcium values.
IV Infusion Site Errors: Samples taken near an infusion site will be diluted. Results for most tests will be low, except for those in the infusion fluid (e.g., saline infusion leads to low overall results but raised sodium and chloride).
Analytical and Transcription Errors: Rare but possible. Transcription errors are decreasing due to electronic downloads. Most internal laboratory errors occur at reception due to mislabeling during sample intake.

Specimen Collection and Biological Fluids

Biological Fluids: Includes blood, urine, saliva, sputum, faeces, tissue/cells, cerebrospinal fluid (CSF), peritoneal fluid, synovial fluid, pleural fluid, and stones.
Blood Specimens: The most frequently used fluid in clinical biochemistry. * Venous Blood: Most common; usually drawn from a vein in the front of the elbow. * Capillary Blood: Small volumes ( $< 0.2\,ml$ ) taken from a finger or thumb. * Arterial Blood: Drawn under local anesthesia for blood gas determinations.
Precautions: Use of sterile/disposable needles/syringes, cleaning the skin, and using clean, dry collection tubes.

Blood Composition and Coagulation Physiology

Types of Blood Specimens: * Whole Blood: Often mixed with anticoagulants. Used for pH, glucose, urea, hemoglobin, pyruvate, and lactate. * Plasma: Liquid portion of whole blood (acellular, $55\%$ ). Contains water ( $91\%$ ), proteins ( $8\%$ ), and other solutes ( $1\%$ ). * Serum: Fluid remaining after blood clotted and is centrifuged. Lacks fibrinogen. * Red Blood Cells (RBCs): Used for determining abnormal hemoglobins and enzymes like G6PD.
Hemostasis Stages: * Primary Hemostasis: Characterized by blood vessel constriction (vasoconstriction) and platelet aggregation at the injury site to form a sticky plug. * Secondary Hemostasis (Coagulation): The process of forming a stable fibrin clot.
Clotting Factors: There are $13$ principal factors assigned Roman numerals ( $I$ to $XIII$ ).
Coagulation Pathways: * Extrinsic Pathway: First pathway activated; stimulated by Tissue Factor (released by cells external to vessels) activating Factor $VII$ . * Intrinsic Pathway: Activated by internal vessel injury; begins with the activation of Factor $XII$ (Hageman factor). * Common Pathway: Both pathways converge at Factor $X$ activation, leading to thrombin production and the conversion of soluble fibrinogen to insoluble fibrin.

Anticoagulants and Testing

In Vivo Anticoagulants: * Parenteral: Heparin, Low molecular weight heparins, heparinoids. * Oral: Coumarin derivatives (e.g., Warfarin), Indandione derivatives.
In Vitro Anticoagulants: Heparin, Sodium citrate, sodium oxalate, sodium edetate (EDTA).
Anticoagulant Testing: * PT (Prothrombin Time): Measures time for a fibrin clot to form and assesses clotting activity. * INR (International Normalized Ratio): Compares patient PT results to a normal average to ensure therapeutic doses of oral anticoagulants.
Targeted Inhibitors: * Factor Xa Inhibitors: Apixaban, Betrixaban, Edoxaban, Rivaroxaban. * Direct Thrombin Inhibitors: Argatroban, Dabigatran.

Laboratory Tube Selection (Vacutainers)

Light-blue: Contains $3.2\%$ Sodium citrate. Prevents clotting by binding calcium. Used for Coagulation tests.
Red or Gold: Serum tube with or without clot activator/gel. Used for Chemistry, serology, and immunology.
Green: Contains Sodium or lithium heparin. Inhibits thrombin and thromboplastin. Used for Stat and routine chemistry.
Lavender or Pink: Contains Potassium EDTA. Binds calcium. Used for Hematology and blood bank.
Gray: Contains Sodium fluoride and oxalate. Fluoride inhibits glycolysis. used for Glucose, blood alcohol, and lactic acid.

Age-Specific Reference Ranges: PSA Example

Prostate-Specific Antigen (PSA) Reference Values (ng/mL): * 40-49 years: Asian ( $0-2.0$ ), African American ( $0-2.0$ ), Caucasian ( $0-2.5$ ). * 50-59 years: Asian ( $0-3.0$ ), African American ( $0-4.0$ ), Caucasian ( $0-3.5$ ). * 60-69 years: Asian ( $0-4.0$ ), African American ( $0-4.5$ ), Caucasian ( $0-4.5$ ). * 70-79 years: Asian ( $0-5.0$ ), African American ( $0-5.5$ ), Caucasian ( $0-6.5$ ).