Interpreting Test Results

Question 1

What % of diagnoses depend on laboratory tests>

How many different tests are available?

Answer 1

Around 70% of diagnoses depend on laboratory tests

There is around 1200 tests available

Question 2

What is meant by “chemical pathology”?

Answer 2

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

Question 3

What are the major interests of chemical pathology?

Answer 3

1. Physiological systems
2. Homeostasis
3. Endocrinology
4. Metabolism

Question 4

What are the stages involved in the core clinical process?

Answer 4

Question 5

What are the contents of a lab report?

Answer 5

1. Demographic details
2. Results from laboratory analysis
3. Reference ranges
4. Comments and advice

Question 6

What is meant by “the normal range”?

Answer 6

Reference range

This defines the values of a biochemical test found in healthy subjects against which patient values can be compared

Question 7

How is the reference range defined?

Answer 7

The reference range is within 2 standard deviations either side of the mean

Question 8

Why does the ideal diagnostic test not exist?

Answer 8

An ideal diagnostic test would have no false positives or negatives

Question 9

Why are ideal tests rarely available in routine practice?

Answer 9

High sensitivity and specificity rarely exist

increased sensitivity is traded for decreased specifity and vice versa

Question 10

What are the 3 strategies to improve tests?

Answer 10

1. Select a more appropriate “normal” population
2. Use a combination of tests
3. Combine tests to achieve a diagnostic goal

Question 11

What characteristics define an appropriate “normal” population?

Answer 11

1. Patients with similar presenting symptoms
2. Same age
3. Same gender
4. Same underlying disease

Question 12

What is meant by setting cut-offs for sensitive screening tests?

Answer 12

Adjust the limit to ensure that there are no false positives or negatives

Question 13

How are cut-offs set for specific second line tests?

Answer 13

Question 14

Which factors tend to affect reference ranges?

Answer 14

• 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

Question 15

What is meant by diurnal rhythm of cortisol?

Answer 15

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

Question 16

What is meant by the glucose tolerance test?

Answer 16

a blood test used to diagnose gestational diabetes

it tests the body’s ability to maintain a normal blood glucose level

Question 17

What are the normal limits for the glucose tolerance test?

Answer 17

The cut-off for gestational diabetes is:

> 11.1 mmol/L after 2 hours

and/or

> 7.8 mmol/L at 0 hours

Question 18

What action limits and therapeutic ranges are used to trigger therapeutic/investigative actions?

Answer 18

Action limits:

• paracetamol
• cholesterol

Therpeutic ranges (for drugs):

• lithium
• digoxin

Question 19

Why are there action limits for cholesterol?

Answer 19

The relative risk of CHD increases exponentially as cholesterol levels increase

Question 20

What is Phenytoin used for?

How is it monitored?

Answer 20

It is an anti-seizure drug

There is a reference range in which the concentration of drug should remain in

Question 21

What are the action limits for paracetamol like?

Answer 21

Treatment is desirable to prevent liver damage, but it is only effective within a certain reference range

Question 22

What 3 things must be remembered when interpreting biochemical values?

Answer 22

1. Biochemical measures must be made under appropriate conditions
2. Any biochemical value must be interpreted in the light of an appropriate reference range
3. An understanding of normal physiological regulation is assumed

Question 23

What is the equation for protein glycosylation?

Answer 23

Protein + glucose = advanced glycosylated end-products (AGEs)

Question 24

What is type of process is protein glycosylation?

In which types of tests is it used and why?

Answer 24

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

Question 25

What is the rate of formation of AGEs proportional to?

Answer 25

1. Glucose concentration
2. Time

Question 26

What is HbA1c?

What does its % concentration suggest?

Answer 26

It is a stable glycosylated haemoglobin

its percentage concentration indicates cumulative glucose exposure