Introduction to the Clinical Biochemistry Laboratory

examples of analytical techniques

• measurement of enzyme activities

• spectrophotometry

• electrophoresis

• separation of molecules based on physical characteristics

• immunoassays

• HPLC

What type of testing uses reference ranges?

quantitative

What is discretional or selective requesting?

Tests ordered based on the patient’s signs, symptoms and history.

Based on the individual patient and situation.

Can be core or specialised

Why are biochemical tests carried out?

• to confirm diagnosis

• to aid differential diagnosis

• to assess disease severity

• to monitor progress

• to monitor therapy

What is the difference between plasma and serum?

• Serum- blood clots naturally, remaining liquid is serum (contains no anticoagulants)

• Plasma- blood mixed with anticoagulant (blood minus cells)

types of collection tubes

What is the generic tube used for serum? What does it contain?

yellow tube

SST gel (separates serum from blood cells)

What is the tube used for glucose? What does it contain?

grey tube

fluoride oxalate

for glucose, lactate, alcohol

types of biochemistry profiling tests

1. renal function

2. liver function

3. thyroid function

4. bone profile

5. lipid profile

examples of preanalytical factors in specimen collection

• time of day

• fasting or random

• posture of patient

• catheters/drips

• sample collection tube

• sample volume

• sample labelling

Give three conditions of serum/plasma after centrifugation that can affect testing?

• haemolysis

• icterus

• lipaemia

What is haemolysis and what causes it?

Bursting of RBCs causes contents to be released (e.g. Hb, AST, K+, LDH, ALP)

Caused by:

• inappropriate venepuncture technique

• overvigorous mixing

• prolonged storage before centrifugation

• inappropriate anti-coagulant used

Why are haemolytic samples a problem?

Hb has maximum absorbance peak at 415nm.

Interferes with readings if other tests are carried out at that same wavelength.

What is icterus and why is it a problem?

high concentration of bilirubin in blood

occurs in acute liver disease

problematic because can interfere if other analytes are measured at same wavelength as bilirubin (450-460nm)

What is lipaemia and why is it a problem?

high concentration of triacylglycerols (TAGs)

cloudy sample

reduces transmission of light, interferes with spectrophotometric tests

What is a QMS?

quality management system

• set of procedures, processes and practices that ensures a laboratory‘s operations are consistent, reliable and accurate

four components of QMS

1. quality planning (inventory, purchasing)

2. quality control (IQC daily monitoring, Westgard rules)

3. quality assurance (EQA schemes)

4. quality improvement (audit)

What is a Levey-Jennings chart and how is it interpreted?

determines whether IQC is valid (usually within 2sd)

interpreted using Westgard rules (usually 2,2s- reject when 2 consecutive controls exceed the same mean + or - 2 sd)

What are the two types of analytical variation?

1. systematic (precise)

2. random (accurate)

common sources of random variation

• difference in operator technique

• sample handling (haemolysis, icterus, lipaemia, etc.)

• environmental changes during testing

common sources of systematic variation

• equipment malfunction

• expired reagents

biological sources of variation within and between individuals

within individuals (diet, time of day posture, exercise, menstrual cycle)

between individuals (age, race, ethnicity, BMI)

What are reference ranges?

represents 95% of healthy individuals

“reference” not “normal” (5% of healthy individuals lie outside)

What is an alternative to reference ranges?

target values

e.g. cholesterol

What is sensitivity?

How effective is the test at providing a positive result in the presence of disease?

TRUE-POSITIVES (TPs)

What is specificity?

How effective is the test at providing a negative result in the absence of disease?

TRUE-NEGATIVES (TNs)

PPV (positive predictive value)

The probability that a person with a positive test result actually has the condition (% of TPs)

PPV (%) = [TP/TP+FP] (x 100)

NPV (negative predictive value)

The probability that a person with a negative test result actually doesn’t have the condition (% of TNs)

NPV (%) = [TN/TN+FN] (x 100)

POCT pros and cons

What are the two main types of screening programmes?

1. well-population screening- proactively testing large groups of apparently healthy people to find those at higher risk for diseases.

2. case-finding programmes- targeted at individuals suspected to be at risk or already showing vague signs.

examples of case-finding screening programmes