Analytical Matrices and Common Interferences

Question 1

What are factors that can influence test results?

Answer 1

Disease

• Genetic
• Environmental

Patient Factors

• Pathology
• Physiology

Sample Factors

• Pre-analytical
• Analytical (The Sample Matrix)

Question 2

What is the Matrix?

Answer 2

Refers to the components of a sample other than the analyte.

Question 3

Why is the Matrix important?

Answer 3

The matrix can have a considerable effect on the way the analysis is conducted and the quality of the results obtained; such effects are called matrix effects

Question 4

What are matrix effects?

Answer 4

• The combined effects of all components of the sample other than the analyte on the measurement of the quantity

Question 5

How is a Matrix different to Interference?

Answer 5

If a specific component can be identified as causing an effect then this is referred to as ‘interference’

Question 6

What is the composition of Blood?

Answer 6

• Serum / plasma (~55 to 65%)
• White cells, platelets etc ‘Buffy Coat’
• Packed Red Blood Cells (~ 35 to 45%)

Question 7

What are differences beteeen plasma and serum?

Answer 7

• Plasma is foemd from unclotted blood
• Serum is collected from collected blood

Question 8

What are examples of Preservatives used for collecting blood samples?

Answer 8

• Potassium EDTA
• Lithium Heparin
• Sodium Citrate
• Fluoride oxalate

Question 9

What are some common interferences?

Answer 9

• Haemolysed blood
• Lipaemic blood
• Icteric

Question 10

How does haemolysis occur?

Answer 10

• Patient: In vivo haemolysis
• Phlebotomy: Needle Gauge, tourniquet time, tube type
• Specimen transport: Pneumatic tube, time of transport, storage prior to transport
• Processing: Time between collection and centrifugation

Question 11

Why is Blood preferred in Lab tests?

Answer 11

1. More convenient than Tissue Biopsies
2. Blood Travels Around the Body
3. Can collect at specified times
4. Can use to detect

• Chemicals Leaking from Damaged Tissues
• Hormones Travelling from Gland to target tissues
• Waste not Cleared by Kidneys
• Metabolites not being Controlled Properly
• Deficient Nutrients

Question 13

Why is Urine used in Lab tests?

Answer 13

• Mostly water (>95%), depends on diet, time of day, climate, physical activity and body size
• Natural waste product
• Easy to obtain
• 24 hr urine collection
• Preservatives/special collection tubes
• Temperature (solubility)

Question 14

What can urine be used to measure?

Answer 14

• Electrolytes
• Nitrogenous compounds
• Phosphates
• Drug metabolites

Question 15

How can urine be unreliable?

Answer 15

• Preservatives (Warning for patients)
• Special containers e.g. Copper
• Certain time of day
• Concentration step
• Pre-analytical clean-up e.g. for drug analysis

Question 16

How are faeces made up?

Answer 16

• Complex mixture of food residues, secreta of the GI tract, desquamated cells of the GI wall and constituents of the intestinal flora
• Exact composition is very inconsistent, and depends on diet, intestinal luminal function, water intake, infection etc
• Highly variable water content (from ~30% to ~95%)

Question 17

Describe the chemistry of faeces?

Answer 17

• Hyperosmolal compared to blood (typically 280 – 500 mOsmol/kg)
• Acidic relative to blood (pH ≤ 6 usually)
• Also contains Electrolytes and Nitrogenous compounds

Question 18

Describe CSF

Answer 18

• Secretory product of: Ventricles, Choroid plexus, Structures of the cerebral subarachnoid space
• Slightly hyperosmolal compared with blood (~290 to 324 mOsmol/kg)
• Levels of typical components such as urea, creatinine, sodium and potassium are similar to those found in blood or plasma
• Very low total protein levels (200 to 400 mg/L)

Question 19

What has to be taken into account in the analysis of CSF?

Answer 19

• How many aliquots?
• Which department gets it first?
• Microbiological contamination
• Contamination from blood
• Protection from light

Question 20

What are some positive of dried blood spots?

Answer 20

• Less invasive
• Low pain – especially good in infants, elderly
• Stable, no cold-chain required
• Amenable to field studies

Question 21

What are negatives of Dried Blood Spots?

Answer 21

• Relationship between spot punched disc and blood volume can be variable – spot quality
• Calibration challenges
• Interference from filter paper fibres and other manufacturing components

Question 22

How is matrix matching done?

Answer 22

• Best practice to use calibrators made up using a diluent that is as close in composition as possible to the matrix in which you want to measure
• Synthetic versions of some physiological fluids are commercially available such as saliva and urine

Question 23

What can be done when a matrix does not contain the compound that you are interested in?

Answer 23

Try:

• ‘Stripped’ plasma e.g. Charcoal; Antibodies etc
• Calibrators diluted in 145 mmol/L saline containing 1 or 2% Human or Bovine Serum Albumin

Question 24

What issues with the analytical matrix?

Answer 24

• Diluents: Water, Saline, BSA, stripped serum, “0” calibrator
• Precipitation, dialysis etc.
• Calibrators & Controls
• Co-factors, Activators, Inhibitors
• Post-mortem bloods -may need clean-up
• Autoantibodies, heterophilics etc.
• Chromatic Interference: serum indices (Lipaemia, Haemolysis, Icterus
• Temperature, pressure, haematocrit (ABGs)
• Drugs - in vitro interference
• Dietary additives - in vitro interference