Errors Requesting appropriate tests Writing prescription Reading prescription Sample collection Sampling times environmental factors Drug interferences Patient identification Sample transfer Technician errors Instrumental errors Method limitations Data entry mistakes Interpretation of results How can we trust
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In fluorescence this can lead to difficulties with fluorescers with a small Stokes shift. Fluorescence may not be easy to resolve from the exciting wavelength.
Another problem is associated with scattering of the incident light to the detector, especially when samples are somewhat turbid.
Why ELISA cant be fully automated
• Can you tell the difference between how many marks are in each box?
400 360Sensitivity of Absorbance Measurements
Spectrophotometry: Luminescence
Sensitivity of Luminescence
Measurements
• Can you tell the difference between how many marks are in each box?
0 40
Spectrophotometry: Luminescence
Immunoassay Signal Generation
10-14 10-15 10-16 10-17 10-18 10-19
ENZYME COLOUR DETECTION
ENZYME FLUORESCENCE DETECTION
TIME RESOLVED FLUORESCENCE DETECTION
RADIOACTIVITY (I125)
CHEMILUMINESCENCE DETECTION
Minimum Detection Limit (moles of tracer)
Det
ecti
on
Me
tho
d
Sensitivities of different tracers
VITROS ECi DETECTION SYSTEM Vitros ECi
What you want
• Accuracy
• Precision
• Speed
• Sensitivity
• Specificity
Immunoassays:
• Radioimmunoassay (RIA, IRMA)
• Elisa (enzyme linked immunosorbant assay)
• Immunofluorometric assay
• Immunluminometric assay
Immunoassays basics
(virus?)
Physical Concepts in immunoassays:
• Radioactivity
• Spectrophotometery
• Fluorescence
• Luminescence
• The wavelength of light determines how it interacts with matter.
• We use these interactions as a probe to obtain chemical information about samples.
• Spectrophotometry is the use of “light” in chemical measurements
Spectrophotometry
400 nm 750 nm
IRUV
IR typically referred to by wavenumber (10-12,500 cm-1)
Blue - OrangeBlue-green - RedGreen - Purple
Fluorescence
FITC + UV light
Phosphorescence
Chemiluminescence
Bioluminescence
Antigen
HRP label
2nd Antibody
Immunometric Assay on Streptavidin Coated Well
Substrate
Light
1st Antibody
Biotin tagSolid PhasePlastic Well
Streptavidin
Biotin spread evenly over well
Ferritin Antibody: Passive Adsorption
Ferritin Antibody: Biotinylated on Vitros Coated Well
Basic requirements for immunoassay
• Standards
• Specific antibodies
• Labelled antigen or antibody
• Separation system
• Quality control
Advantages of 2-site immunometric assays
• Increased sensitivity
• Increased precision
• Better specificity
• Greater assay range
• Shorter assay times
Disadvantages of 2 site immumetric assays
• Need for large quantities of pure antibody (monoclonal antibodies usually employed)
• 2 antibody binding sites required (limit range of analysis)
• High dose “hook” effect• Need for multiple washing steps• Non specific interference due to heterophyllic
antibodies
Advantages of isotopic labels
• Simple coupling reactions
• Label properties do not alter on coupling
• No background signal
• Efficient/convenient detection systems
• No additional cost to detect signal
• Very useful for research assays
Non isotopic labels - advantages
• No radioactivity– safety aspects– disposal
• Extended life of label• Speed of detection• Ease of automation• Theoretical increase in sensitivity• Possibility of homogeneous assays• Simple/safe label preparation
Non isotopic labels - disadvantages
• Safety aspects of labels/substrates
• Serum/buffer effects
• Extra manipulations in detection
• Inefficient detection in some cases
• No recounting possible in some systems
• Limitation of separation system
• “Dedicated” instruments
• Commercial pressures
Hormone assay in the future
• Dominated by immunoassay techniques• ? GCMS
• Increased sensitivity• Better automation
– computers– robotics– non isotopic labels
• Near patient testing devices
Immunoassay automation
• Non isotopic labels
• Microprocessor power
• Improved robotics
• Better antibodies faster reaction times
Immunoassay Analysers
• Immunological reagents
• Precision usually good
• Wide variations in sensitivity, specificity and accuracy between analysers
• Careful definition of assay requirements
• Whether any one analyser will satisfy all requirements
Types of automated system
• Work simplified batch systems
• Automated batch systems
• Total automation (black box) - random access
• Portable (bedside biochemistry) instruments
Intellicheck Technology – advanced instrument intelligence - monitors and verifies:
Patient sample qualitydetects short samples, clots, bubbles, viscosity
Sub-system performance Sample & reagent volumes in reaction well Reports issues to the operator