Interpreting Test Results Flashcards
What % of diagnoses depend on laboratory tests>
How many different tests are available?
Around 70% of diagnoses depend on laboratory tests
There is around 1200 tests available
What is meant by “chemical pathology”?
A discipline involved with diagnosis, screening, monitoring and therapeutic control
They perform diagnostic tests and provide advice
It also involves research into the biochemical basis of disease and provides the science of diagnosis
What are the major interests of chemical pathology?
- Physiological systems
- Homeostasis
- Endocrinology
- Metabolism
What are the stages involved in the core clinical process?
What are the contents of a lab report?
- Demographic details
- Results from laboratory analysis
- Reference ranges
- Comments and advice
What is meant by “the normal range”?
Reference range
This defines the values of a biochemical test found in healthy subjects against which patient values can be compared
How is the reference range defined?
The reference range is within 2 standard deviations either side of the mean
Why does the ideal diagnostic test not exist?
An ideal diagnostic test would have no false positives or negatives
Why are ideal tests rarely available in routine practice?
High sensitivity and specificity rarely exist
increased sensitivity is traded for decreased specifity and vice versa
What are the 3 strategies to improve tests?
- Select a more appropriate “normal” population
- Use a combination of tests
- Combine tests to achieve a diagnostic goal
What characteristics define an appropriate “normal” population?
- Patients with similar presenting symptoms
- Same age
- Same gender
- Same underlying disease
What is meant by setting cut-offs for sensitive screening tests?
Adjust the limit to ensure that there are no false positives or negatives
How are cut-offs set for specific second line tests?
Which factors tend to affect reference ranges?
- Age
- gender
- diet
- pregancy
- time of the month
- time of day
- time of year
- weight
- stimulus
These need to be kept in mind when interpreting test results
What is meant by diurnal rhythm of cortisol?
There tends to be a rise in cortisol over night and a peak within one hour of waking
Cortisol levels drop quickly in the morning, increase after the mid-day meal and decline slowly for the rest of the day
What is meant by the glucose tolerance test?
a blood test used to diagnose gestational diabetes
it tests the body’s ability to maintain a normal blood glucose level
What are the normal limits for the glucose tolerance test?
The cut-off for gestational diabetes is:
> 11.1 mmol/L after 2 hours
and/or
> 7.8 mmol/L at 0 hours
What action limits and therapeutic ranges are used to trigger therapeutic/investigative actions?
Action limits:
- paracetamol
- cholesterol
Therpeutic ranges (for drugs):
- lithium
- digoxin
Why are there action limits for cholesterol?
The relative risk of CHD increases exponentially as cholesterol levels increase
What is Phenytoin used for?
How is it monitored?
It is an anti-seizure drug
There is a reference range in which the concentration of drug should remain in
What are the action limits for paracetamol like?
Treatment is desirable to prevent liver damage, but it is only effective within a certain reference range
What 3 things must be remembered when interpreting biochemical values?
- Biochemical measures must be made under appropriate conditions
- Any biochemical value must be interpreted in the light of an appropriate reference range
- An understanding of normal physiological regulation is assumed
What is the equation for protein glycosylation?
Protein + glucose = advanced glycosylated end-products (AGEs)
What is type of process is protein glycosylation?
In which types of tests is it used and why?
It is a non-enzymatic process
it may explain the long-term complications of diabetes
it is the scientific basis for commonly used monitoring tests
What is the rate of formation of AGEs proportional to?
- Glucose concentration
- Time
What is HbA1c?
What does its % concentration suggest?
It is a stable glycosylated haemoglobin
its percentage concentration indicates cumulative glucose exposure
