1. USE OF CLINICAL LABORATORY
Lecture 3

2. 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

3. 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.

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

5. Case history
Investigation report of a person attending diabetic clinic:
Potassium 12.2 mmol/L ( mmol/L)
Sodium 140 mmol/L ( mmol/L)
Calcium 0.34 mmol/L ( mmol/L)
Phosphate 1.22 mmol/L ( mmol/L)

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

7. 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)

8. 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

9. 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.

11. 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

12. Quality assurance
Internal quality control
External quality control

13. 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

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

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

18. 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)

19. 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)

20. + - 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%