USE OF CLINICAL LABORATORY Lecture 3

Sampling errors Blood sampling techniques Hemolysis high potassium contents and other red cell constituents. Prolonged stasis during venepuncture Water will diffuse from vessel into interstitial spaces so concentrated specimen. Proteins and protein bound like thyroxine, ca Insufficient specimen Impossible to perform many tests on small volume

Incorrect specimen container. Errors in timing. 24 hr urine collection Incorrect specimen container. Glucose should be taken in fluoride bottle Samples having EDTA would have markedly reduced ca level Inappropriate sampling site. Should not take blood from limb having I/V line.

Incorrect sample storage Old samples with more than 24 hrs will have high potassium, phosphate and red cell enzymes (LDH)

Case history Investigation report of a person attending diabetic clinic: 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)

Specimen was taken in potassium fluoride and oxalate. (Glucose) Transferred this into citrated tube (ca oxalate insoluble)

Interpretation of results 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. Enzyme activity is expressed as units/L Large molecules like proteins mass units (gm/ L)

Variations in results Analytical variations Biological variations Function of analytical performance Pre analytical (sampling errors) Anlytical (within laboratory human or instrumental) Post analytical Biological variations Within subject variations over time Subject to subject variation within a population

Laboratory analytical performance 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.

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

Quality assurance Internal quality control External quality control

Reference interval The reference interval principle is usually to take the central 95% of a reference population 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

Biological variations Age Sex Diet Timing Stress and anxiety Posture of the patients Effects of exercise Medical history (obesity or diabetes, drug)

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

Sensitivity Sensitivity (Se) = TP / (TP + FN) 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)

Specificity It is the measure of the incidence of negative results in a person 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)

+ - Disease Test Sensitivity = 48/50 = 96% Specificity = 47/50 = 94% 3 51 2 47 49 50 100 Sensitivity = 48/50 = 96% Specificity = 47/50 = 94%