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Laboratory tests

Print Print | Email  Email article to a friend | Last updated: August 22, 2012

Introduction to laboratory tests

Laboratory tests seek to answer questions raised by doctors after talking with and examining the patient.  For example:  How can I confirm the diagnosis?  Are related illnesses present?  Is the treatment working?  Are side-effects present?

Laboratory tests are usually carried out on samples such as blood, dried blood spots on a card, saliva, single urine samples and 24-hour urine collections.  The type of sample will depend on what is being measured, the accuracy required and/or the age of the patient.  Once the sample is taken, the patient does not need to be present for the results to be produced.

The results of hormone tests when healthy vary in response to natural pressures such as food, drink, rest, exercise and the menstrual cycle, which can make results in health and disease overlap.  In ‘dynamic tests’ (where two or more samples are taken over a set time period) doses of hormones, drugs, glucose and natural pressures such as exercise or restriction of water intake, are used to control the influences on results and make them more predictable.

Is there anything the patient needs to do to prepare for having tests taken?

The doctor will advise the patient on how to prepare, because it depends on the particular test.  Most preparation is obvious once it is understood that hormones keep the body working and levels change to meet various demands such as fasting, eating, thirst, smoking, exercise, posture, time of day, stage of the menstrual cycle and medicines.

What should the patient expect when they visit the hospital for a test? 

Bring a fully filled-in request form (the doctor’s responsibility).  It is important to make sure that the patient’s name, date of birth, and date and time of collection are written on the container(s) if urine or faeces has been collected.

Most samples can be taken in the family doctor’s surgery.  However, some hormones last such a short time in the sample that the test must be taken in hospital.  Most blood tests in hospital are taken in a ‘phlebotomy unit’ without having to make an appointment, although some, such as a glucose tolerance test, may require an appointment.

’Dynamic tests’ are more detailed and usually involve resting in bed while blood samples are taken over a period of time.  An overnight stay is not usually needed and patients should come with details of their medicines and prepare as instructed, for example, without having eaten breakfast. 

Any queries about the details of individual tests should be discussed with the patient’s doctor.

Are there any other factors that might affect the outcome of the test?

The blood level of several hormones changes significantly with the time of day.  For example, cortisol and testosterone are highest in the early morning. The response of glands to hormones given to patients during dynamic tests may also show this diurnal variation; for example, the response of the adrenal gland to synacthen is higher in the morning.

The day (stage or phase) of the menstrual cycle also has a major impact on hormone levels.  In general, blood samples are best taken in the ‘first half’ of the cycle (the follicular phase) when normal and abnormal hormone levels are more clearly separated.  However, progesterone may be deliberately measured on day 21 in the middle of the ‘second half’ (luteal phase) to see if ovulation has occurred.

It is human nature to ignore doctors’ advice.  Not taking medication as prescribed, or taking extra the week before the test in an effort to make up for doses missed previously, will give misleading results and the patient may miss out on a full return to health.

The doctor keeps talking about reference ranges.  What does this mean?

A reference range is the range of results seen in healthy people under standard conditions eg, rested, fed or fasted, at nine in the morning or non-pregnant.  There is more than one range to allow for normal variations such as age, gender, the stage of the menstrual cycle, menopause and pregnancy.

Unfortunately, the range for test results in healthy people and those with illnesses usually overlap, so there is rarely a clear dividing line.  A result can be outside the reference range in a healthy person but within it for someone who is ill.  There also needs to be a balance between use of the range for detecting disease (sensitivity) and the need to minimise false diagnosis.  Thus, the reference range includes most results seen in normal people, but not all of them (the extreme ones).  Because of this, the reference range is a guide and not a rigid definition of normality.

There is a similar test advertised on the internet?  Can this test be taken at home?

Samples of urine, saliva, faeces and finger-prick blood, which can be collected at home, are currently used by the NHS for specific tests, especially in relation to children.

It is recommended that all tests are overseen by the patient’s GP and/or endocrinologist.  This is because it is important that a doctor has carried out a full examination of the patient and talked to them face-to-face to ensure that they can properly interpret the results of any tests.  The advantage of a professionally collected sample (by the patient’s GP or endocrinologist), eg, blood from a vein, is that enough is taken to allow double-checking of extreme results, repeating the analysis after dilution, investigating possible human or biological interference, allowing additional tests to be carried out if required without the delay and discomfort of taking another sample.

Some internet laboratories are acceptable, but the small volume of self-taken samples creates limitations.  With hormones, it is often best to measure more than one at once in order to get the ‘full-picture’.

It can be difficult for internet laboratories without full knowledge of individual medical details, for example, the drugs being taken, to appreciate the significance of the results, or fully interpret them.

It is important to take care when comparing results from different laboratories because significant variation can arise from differences in the measurement technology.

How and when will the patient find out the results?

Results are returned to the doctor or specialist nurse who is looking after the patient. They may be contacted immediately to discuss the results or wait until they are next seen in clinic.

Results must be interpreted in the context of other medical details.  With endocrinology in particular, the same numerical result may signify health in one person, but a problem in another.  Patients should discuss results with their doctor rather than trying to interpret them themselves.

Routine results are produced within one working day providing additional measurements are not required, for example, if it has to be double-checked.  Results on samples sent to reference laboratories for special tests may take one to four weeks to return.