Chemical pathology VI - Interferences in lab test Flashcards
Define interference
Effect of a substance present in the sample that alters the correct value of the result,
expressed as concentration or activity for an analyte
Effects of interference in lab tests?
False results > misinterpretation > wrong treatment
Hb concentration is estimated by which index?
Main source of interference of Hb analysis?
How to prevent?
Hemolysis index
In vitro hemolysis = main source of interference
» release intracellular LDH, K, Mg to extracellular fluid, specific levels at specific Hb conc.
Hemolysis occurs in sample collection/ transport:
- careful handling
3 ways high lipid causes chemical analysis interference?
Chylomicrons cause turbidity, light scattering and volume displacement
Partitioning of non-polar compounds
Explain how pseudohyponatremia is linked to high serum lipid or protein?
Pseudohyponatremia caused by hyperlipidemia and hyperproteinaemia
lipids and proteins in blood are barely soluble and precipitates > these lipids and proteins in high amount can occupy space in the plasma
If 10mL of plasma is extracted and high lipid and protein occupies space (e.g. 2mL of space), then the remaining 8mL contains a lower mass of Na than anticipated in 10mL, even though the concentration of Na is the same
Hyperproteinaemia e.g. multiple myeloma»_space; lots of protein in antibodies
Hyperlipidemia e.g. excess chylomicrons
Difference between direct and indirect ISE?
Direct and indirect ion selective electrodes
Direct ISE = direct measurement of serum and plasma ion (Na)
Indirect ISE = measure serum and plasma ion After dilution. Ion level is lower because serum lipid or protein occupies significant space before dilution
3 ways bilirubin causes chemical analysis interference?
- Anti-oxidant»_space; interferes with peroxidase catalyzed reactions (e.g. glucose, cholesterol, uric acids)
- Absorption spectrum with multiple peaks (major at 421nm)»_space; interfere with other chemical spectrums
- Direct chemical reaction with some reactants
2 ways that paraproteins cause chemical analysis interference?
Paraprotein = monoclonal Ab
1) Increases serum viscosity and affects sampling
2) Binds enzymes and forms macro-molecules (e.g. bind to creatine kinase)
Cephalosporin interference.
- What condition?
- What test?
- Consequence?
Cephalosporin can be mistaken as Creatinine in Jaffe test ( colorimetric method used to determine creatinine levels in blood and urine)
e.g. Cephalosporin given to treat renal infection»_space; increase creatinine reading might be false indicator of increased GFR and aggravated infection»_space; False alarm
Glucose oxidase method.
- Name one chemical that causes interference?
Ascorbic acid»_space; Negative interference on glucose oxidase method
5 ways to tackle lab test interferences? ***
1) Delta check: compare current and previous results, half-life and biological changes of reactants and samples
2) Check lab test incompatible with clinical conditions
3) Check Contradictory results/ unusual reaction rates
4) Choose another method with no interference (e.g. Change Jaffe reaction to enzymatic reaction in cephalosporin- creatinine interference)
5) Specific methods to block interference (e.g. block HAMA, PEG precipitation to filter macro-prolactin)
Interference from time of reaction?
Reactions have variable rates
Always allow time to reach steady state
Immunoassay interferences in a double antibody sandwich assay?
1) Interfering or cross-reacting substances:
- bind analyte
- Bind labeled detector antibody
- Bind capture antibody (fixed)
- bridge 2 antibodies without analyte
2) Heterophilic antibody:
- bridge 2 antibodies without analyte
- Bind capture antibody
- Bind labeled detector antibody
3) Poor antibody reactions:
- 2 antibodies bridge without analyte
- Labeled detector antibody fail to bind analyte
Explain how cross-reactivity occurs.
Give one example
Antibodies recognize and bind to cross reactant and original analyte, both share similar structure
e.g. Digoxin immunoassay kits can cross react with digoxin-like immunoreactive substances
What interferences can autoanalyte antibodies produce?
AutoAb that recognizes and binds to target analytes
Cause interference in non-immunoassays and immunoassays:
- e.g. Bind to prolactin/insulin/thyroglobulin and form large Ig complex
- e.g. Bind to enzymes and aggregates to disable enzyme metabolism
Cause of macroprolactinemia?
Prolactin polymerize and aggregates with IgG (autoantibodies) and makes Macroprolactin»_space; false reading of serum hyperprolactinemia without any symptoms
anti-PRL autoantibodies.
10–25% in patients with hyperprolactinemia and 3.7% in general population
D/dx macroprolactinemia and genuine hyperprolactinemia
- Subfractionation with PEG»_space; distinguish true hyperprolactinemia *****
- Dopamine agonist treatment
- Presence of adrenal incidentaloma in normal people without hyperprolactinemia
3 ways that antibodies can cause chemical test interference?
1) Heterophilic Ab»_space; Attacks reagent Ab
(e. g. Human Against Animal Ab HAMA)
2) High-dose Hook effect
3) Aberrant binding with other serum proteins: paraprotein, complement, lyzozymes…etc
Define high dose hook effect
Very high analyte level results in paradoxically low result
Overloaded analytes cause conformational change to Ag-Ab complexes and dissociation, giving abnormally low reading after the maximum equivalence zone
Explain one major source of interference with POCT glucometers.
Peritoneal dialysis: solution contains Icodextrin
Icodextrin metabolized in systemic circulation into glucose polymers (mainly maltose)»_space;> Interferes with GDH-PQQ based Glucometer
> > > Falsely high blood glucose levels
2 methods for POCT glucometers?
Name one interference for each.
Glucose oxidase-based system (interfered by ascorbic acid)
GDH-PQQ based method (interfered by Icodextrin in peritoneal dialysis)
Explain one major interference in Digoxin immunoasay?
Generation of Digoxin-like immuno-reactive substances (DLIS, protein bound) in volume expansion conditions
> > DLIS cross react with anti-digoxin antibodies
Falsely high serum digoxin concentration
List 4 conditions that generate DLIS and gives falsely high serum digoxin readings?
UELP
Volume-expanded conditions
Uremia
Essential hypertension
Liver disease
Pre-eclampsia
How to overcome DLIS interference of Digoxin immunoassay?
Analyze Protein-free ultrafiltrate
Because DLIS are strongly protein-bound
