Hormone measurement • The Problem – very small amounts of hormone in a very complex mixture • Pre-immunoassay – complex and insensitive methods (chemical methods, whole animal or tissue bioassay) – insensitive – imprecise – inaccurate • Immunoassay – first described in 1960 – very rapid expansion since early 1970s – advantages (simplicity, speed, precision, accuracy, sensitivity)
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Hormone measurement
• The Problem– very small amounts of hormone in a very complex mixture
• Pre-immunoassay– complex and insensitive methods (chemical methods, whole
Definitions • Potency estimate: the concentration of the analyte.• Sensitivity: the minimum amount of the analyte
which can be accurately detected.• Specificity: the ability of IA to uniquely measure
the analyte of interest.• Accuracy: agreement between the true answer and
the answer obtained in the IA.• Precision: expressed as inta- interassay variation,
calculated as CV.
Systemic versus random errors
• Systemic: errors deflect repeated measures from true or accurate value.
• Random: are those which primarily affects precision. Random errors can not be eliminated but can be minimized.
Opening definitions
• Is standardisation of Immunoassay different from standardisation of any other types of assay system?
• What do we mean by Standardisation, Calibration?
Do the ‘analytes’ exist?
• Cortisol, Testosterone, Thyroxine can be weighed out, but...
• Free Thyroxine [ie non-protein bound]
• Urinary Free Cortisol[i.e. cortisol that is not conjugated and is in urine]
• Protein/polypeptide hormones
• TSH, hCG, LH and FSH etc.
• Standardisation
Calibration using a measurement standard
• Calibration
set of operations that establish under specified conditions. The relationship between values of quantities indicated by a measuring instrument or measuring system, or values represented by a material measure or a reference material, and corresponding values realised by standards
In broad terms ...
• Standardise = make readings comparable
• Calibrate = make readings correct
• We want to do both!!
Where is immunoassay used
• Medical laboratory – hormones, drugs, tumour markers, specific proteins,
viral antigens, etc
• Point of Care Testing– Drugs, cardiac markers, anticoagulants etc
• Over the Counter– Pregnancy tests, fertility tests etc
• Agriculture, veterinary, pharmaceuticals, research etc
Limited reagent immunoassay
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Include labelled antigen (fixed amount) to indicate the distribution of bound and free analyteCount bound fraction after separation and washing .
• 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
Specialised Immunoassays
• Free hormone assays
• Homogeneous (non separation ) immunoassays
Measurement of free hormones
• ? Free hormones closely reflect the true (active) hormonal state in the body
• In theory, these are an optimal test of hormonal function
• Measurement presents a challenge– very low concentrations– avoiding disturbing equilibrium between bound and
free hormone during measurement– sera from some patients (non-thyroidal illness) contain
interfering substances that can invalidate measurements
Measurement of free hormones
• Reference methods– Initial separation by equilibrium dialysis or
ultrafiltration and measurement of analyte in the separated fraction by immunoassay
– Only minimal dilution of sample possible– Expensive, time consuming, unsuitable for high
volume work– Commercial methods now available
Methods of free hormone estimation
• Index methods (calculation)
• 2 step immunoassay
• Analogue immunoassay
• Labelled antibody assays
Index Method• Correct total hormone concentration for
abnormal binding protein concentration• Measure total hormone (free and bound)
• Measure binding protein (i.e. TBG, SHBG)
• Apply formulae to estimate free hormone
• Simple, rapid, inexpensive
• Variable performance
• Not accurate at very high or low binding protein concentrations
2 Step Immunoassay
• Extract free hormone from serum by adding solid phase antibody
• Wash
• Add label which binds to remaining unoccupied antibody binding sites
Analogue Immunoassay
• Label = analogues which bind to antibody but not to binding proteins (disputed)
• Mix sample, antibody and analogue label– binding of label to antibody is inversely
proportional to free hormone concentration in the sample
Labelled Antibody Assay
• Label = antibody to hormone (usually monoclonal) labelled with 125I or non-isotopic label
• Solid phase = derivative of antigen attached to coated tube or magnetic particles etc
Problems with current free hormone assays
• Reference methods not practical
• Lack of information from manufacturers on kit performance in binding abnormalities
• Frequent changes in kit formulation and methodology
• Large range in concentrations measured in EQAS schemes when comparing different methods
• Patients with same abnormality (ie. non- thyroidal illness) can have normal results with one kit and abnormal results with another kit supposedly based on the same analytical principle
Homogeneous immunoassays
– No separation step therefore simple and easy to automate
– Lack sensitivity
• Fluorescence polarisation
• Turbidimetry/nephelometry/latex agglutination
Reference Range• A guide to the levels expected in normal
people
• “Normal population”– laboratory staff– hospital out patients– occupational health– medical students
• Must take age/sex etc into account
• Must take a sufficient sample ( 100)
• Age– T3 in elderly people
• Sex– Testosterone in males/females
• Time of day– Cortisol
• Time of month– Oestradiol/progesterone (females)
• Diet– Insulin
• Illness– Sick euthyroid
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