Clinical biochemistry (Lec 7) (Set 1)

Clinical Biochemistry Lab

• Also known as chemical pathology and clinical chemistry, applying chemical and biochemical methods to study disease.

Samples

• Tests are needed for diagnosis, monitoring treatment, disease screening, and prognosis.
• Samples are usually serum from venous blood or urine.
• Blood is collected into blood specimen tubes depending on the requested test.
• Serum is obtained by centrifuging blood without an anticoagulant.
• Plasma is obtained by centrifuging blood with an anticoagulant.

Biochemical Analyses

• Other specimens used include urine, arterial blood, faeces, cerebrospinal fluid, sweat, and saliva.

Processing a Request

• Correct specimen and completed clinical biochemistry form are required.
• Appropriate information ensures correct test and patient identification.
• Bar coding ensures correct sample identification.
• Automated analysis provides results with minimum delay.

Clinical Biochemistry Process

• The laboratory process involves receiving the specimen, analyzing the sample, and returning results to the clinician.

Clinical Biochemistry Test Results

• Diseases cause significant changes in body fluid chemical composition.
• Tests measure these changes quantitatively, such as:
  • Raised blood sugar in diabetes mellitus due to lack of insulin.
  • Raised blood cardiac biomarkers due to their release from heart muscle after a heart attack.

Precision and Accuracy

• Precision refers to reproducibility.
• Accuracy refers to how close the measured value is to the actual value.

Analytical Sensitivity and Specificity

• Sensitivity refers to the minimum amount of analyte detectable by the assay.
• Specificity refers to the assay's ability to distinguish between the requested analyte and other substances.

Quality Assurance

• Internal quality assurance involves daily or every-test quality control samples.
• External quality assurance involves distributing identical samples to several labs for comparison.

Reference Intervals/Ranges

• Reference ranges are chosen to include 95% of healthy volunteer values.
• 5% of the normal population will be outside the reference range.
• Reference ranges vary slightly according to source or lab.

False Positives and False Negatives

• False positive: result outside the reference range, but person is healthy.
• False negative: patient has the disease, but gives a 'normal' result.

Biological Factors Affecting Interpretation

• Different reference ranges for genetically male and female individuals, depending on age, pregnancy, diet, time of day, and other factors.

Point of Care Testing

• Rapid reassurance or further investigations convenient and clinically meaningful.
• Range of analytes includes glucose, protein, cholesterol, urea, and creatinine, among others.

Problems with Biochemical Specimens

• Difficult blood sampling, insufficient specimen, errors in timing, and incorrect specimen container or storage can affect results.

Core Biochemical Tests

• Us & Es (Urea and creatinine & Electrolytes): sodium, potassium, chloride, bicarbonate, calcium, and phosphate.
• LFTs (Liver function tests): Alkaline phosphatase; alanine amino transferase; bilirubin; albumin.
• Thyroxine and thyroid stimulating hormone, H+, pCO2, pO2 (blood gases), and glucose.

Specialised Tests

• Specialised techniques (not offered by every hospital) include trace elements, hormones, specific proteins (e.g., tumour markers), and DNA analysis.