Analytical Matrices and Common Interferences Flashcards
What are factors that can influence test results?
Disease
- Genetic
- Environmental
Patient Factors
- Pathology
- Physiology
Sample Factors
- Pre-analytical
- Analytical (The Sample Matrix)
What is the Matrix?
Refers to the components of a sample other than the analyte.
Why is the Matrix important?
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
What are matrix effects?
- The combined effects of all components of the sample other than the analyte on the measurement of the quantity
How is a Matrix different to Interference?
If a specific component can be identified as causing an effect then this is referred to as ‘interference’
What is the composition of Blood?
- Serum / plasma (~55 to 65%)
- White cells, platelets etc ‘Buffy Coat’
- Packed Red Blood Cells (~ 35 to 45%)
What are differences beteeen plasma and serum?
- Plasma is foemd from unclotted blood
- Serum is collected from collected blood
What are examples of Preservatives used for collecting blood samples?
- Potassium EDTA
- Lithium Heparin
- Sodium Citrate
- Fluoride oxalate
What are some common interferences?
- Haemolysed blood
- Lipaemic blood
- Icteric
How does haemolysis occur?
- 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
Why is Blood preferred in Lab tests?
- More convenient than Tissue Biopsies
- Blood Travels Around the Body
- Can collect at specified times
- 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
Why is Urine used in Lab tests?
- 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)
What can urine be used to measure?
- Electrolytes
- Nitrogenous compounds
- Phosphates
- Drug metabolites
How can urine be unreliable?
- Preservatives (Warning for patients)
- Special containers e.g. Copper
- Certain time of day
- Concentration step
- Pre-analytical clean-up e.g. for drug analysis
How are faeces made up?
- 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%)
Describe the chemistry of faeces?
- Hyperosmolal compared to blood (typically 280 – 500 mOsmol/kg)
- Acidic relative to blood (pH ≤ 6 usually)
- Also contains Electrolytes and Nitrogenous compounds
Describe CSF
- 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)
What has to be taken into account in the analysis of CSF?
- How many aliquots?
- Which department gets it first?
- Microbiological contamination
- Contamination from blood
- Protection from light
What are some positive of dried blood spots?
- Less invasive
- Low pain – especially good in infants, elderly
- Stable, no cold-chain required
- Amenable to field studies
What are negatives of Dried Blood Spots?
- Relationship between spot punched disc and blood volume can be variable – spot quality
- Calibration challenges
- Interference from filter paper fibres and other manufacturing components
How is matrix matching done?
- 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
What can be done when a matrix does not contain the compound that you are interested in?
Try:
- ‘Stripped’ plasma e.g. Charcoal; Antibodies etc
- Calibrators diluted in 145 mmol/L saline containing 1 or 2% Human or Bovine Serum Albumin
What issues with the analytical matrix?
- 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
