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USE OF CLINICAL LABORATORY

USE OF CLINICAL LABORATORY. Lecture 2. Blood sampling techniques Difficulty in blood specimen can lead to haemolysis (lysis of red blood cells) with high potassium contents and other red cell constituents. Prolonged stasis during venepuncture

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USE OF CLINICAL LABORATORY

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  1. USE OF CLINICAL LABORATORY Lecture2

  2. Blood sampling techniques • Difficulty in blood specimen can lead to haemolysis(lysis of red blood cells) with high potassium contents and other red cell constituents. • Prolonged stasis during venepuncture • Water from serum will diffuse into the interstitial spaces and the serum/plasma specimen will get concentrated, • Proteins and protein bound components of plasma (calcium or thyroxine) will falsely elevated. • Thyroxineis the hormone secreted by the thyroid gland and its function is to stimulate the consumption of oxygen and thus the metabolism of all cells and tissues in the body. • Insufficient specimen • Impossible to perform many tests with small volume of specimen Sampling errors

  3. Venepuncture, venopuncture or venipuncture is the process of obtaining intravenous access for the purpose of intravenous therapy or for blood sampling of venous blood. Sites of venepuncture Intravenous sampling –     Convenient vein in the forearm–     Basilic and median cubital vein , crease of elbow–     Metacarpal vein–     Veins in arm and forearm Why Venipuncture sites are rotatedTo prevent ?    SCLEROSIS (hardening of tissue) ,THROMBOSIS (blood clot)

  4. Errors in timing. • Biggest source of errors, e.g. 24 hrs for urine collection is critical • Incorrect specimen container. • For many specimens the container should have anticoagulant/ or preservatives, e.g. glucose should be taken in fluoride bottle to inhibit glycolysis, • Exposure of sample to EDTA would have markedly reduced ca level, therefore avoide it • Inappropriate sampling site. • No sample should be taken from DOWNSTREAM an IV drip / arm in which 5% glucose is infused, must spared. • Incorrect sample storage • Generally, a blood sample should not stored overnight to avoid leakage of RBCs components ( false high K. PO4, LDH-ase ) • In other words old samples with more than 24 hrs will have high potassium,

  5. Blood sample of a 65 year old lady was left over by a technician in his car and handed it over to the lab the next morning. • Investigation report was alarming, • Why??? • Potassium 12.2 mmol/L (3.5-5.5 mmol/L) • Sodium 140 mmol/L (135-145 mmol/L) • Calcium 0.34 mmol/L (2.2-2.6 mmol/L) • Phosphate 1.22 mmol/L (0.8-1.4 mmol/L) Case history

  6. Qualitative • Quantitative • Semi quantitative • The concentration of any known analyte in the body compartment is a ratio: the amount of substance dissolved in a known volume. • The amount of analyte can increase or decrease • The volume of fluid can change as well • Enzyme activity is expressed as units/L • E.g. miller units/mL or L • Large molecules like proteins mass units (gm/ L) Interpretation of results

  7. The S.I. units apply to clinical chemistry as follows. • Where the molecular weight of the substance being measured in known, the units of quantity should be the mole_submultiple of a mole. e.g., milimoles and Micromoles. • The units of volume should be the liter. Units of concentration. will there fore be millimoles per liter etc. e.g., sodium of .140 mg/l in S.I. units is 140 m mol/l. glucose of 180 mg/100 ml in S.I. units is 10 m mol/l • 3. When the molecular weight is not known, then for example for serum protein or albumin determinations the concentration should be expressed in grams per liter i.e. 7.0 g/100 ml becomes 70 g/l.

  8. Analytical variations • Are function of analytical performance • Pre analytical (sampling errors) • Analytical (within laboratory human or instrumental) • Post analytical • Biological variations • Changes that take place in patient over time • Subject to subject variation within a population Variations in resultsbiochemical measurements can vary for two reasons.

  9. Precision and accuracy • Precision is the reproducibility of an analytical method • Accuracy is how close the measured value is to the actual value • It is the objective in every biochemical method to provide good precision and accuracy. Laboratory analytical performance

  10. Analytical sensitivity of an assay is a measure of how little of the analyte, a method can detect. • Analytical specificity is how good the assay is at discriminating between requested analyte and interfering agents Analytical sensitivity and specificity

  11. Internal quality control quite frequently performed to assure the performance of instruments and methods • External quality control Similar samples are distributed among different labs and then compared Quality assurance

  12. Analytical variations are less than biological variations. • Biochemical tests are compared to the reference interval, this principle is usually to take the central 95% of a reference healthy population • So 5% of the population will have result outside the reference interval. • the subjects being tested (the reference population) should be as similar as possible to that for which the test will be applied, with the exception of the presence of disease. • 120 is the number of data points needed for a reference interval study • Perform according to usual practice Reference interval

  13. Sex (serum creatinine are different for men and women) • Age (different RI for neonates, children, adults, aged) • Diet (sample taken while fasting or after heavy meal) • Timing (variations in day and night sampling) • Stress and anxiety • Posture of the patients • Effects of exercise (enzymatic release during exercise) • Medical history (infection, obesity or diabetes, drug) • Pregnancy Biological factors affecting test results

  14. How reliable it is • How suitable it is for intended purpose • Sensitivity • Specificity Clinical utility of laboratory tests

  15. It is the measure of the incidence of positive results in a patient known to have the disease. true positive (TP) • Sensitivity (Se) = TP / (TP + FN) Sensitivity

  16. It is the measure of the incidence of negative results in a patient known to be free of disease true negative (TN) • Specificity (Sp) = TN / (TN + FP) • Ideal diagnostic test should be 100% sensitive (positive in all diseased) and 100% specific (negative in all healthy) Specificity

  17. Sensitivity = 48/50 = 96% Specificity = 47/50 = 94%

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