What GPs need to know about self-testing
Sue Jones and Dr Lisa Argent look at whether
over-the-counter monitoring kits can be useful
An increasing number of people are taking a positive approach to their health and a personal approach to monitoring it. This has led to a market for diagnostic kits and many are widely available to purchase over-the-counter. The range of tests varies from allergy to HIV.
With increasing use of the internet, further tests are available, but the robustness of some may be dubious and without clear quality control should be avoided by patients.
From the GP's perspective there is often debate over whether these tests help or hinder. The widespread use of cholesterol testing kits have created the 'worried well' and further clinical investigation often shows their risk factors are very low.
Diagnostic testing involves the use of a variety of tests to help detect a condition in its early stages or alert an individual to a cause for concern. Historically, pharmacists have been involved in diagnostic testing for many years in the form of pregnancy testing and weight measurement. Due to their high street location, pharmacists have more recently become involved in many local initiatives such as screening for diabetes or coronary heart disease1,2.
The various tests that may be performed rely on the measurement of physical, chemical, immunological or combination techniques leading to a qualitative 'yes/no' or a quantitative result.
Ideally, the diagnostic test itself should be accurate and precise and sensitive and specific leading to a correct diagnosis. In practice this is not the case and these factors need to be taken into consideration.
The skill of the tester also comes under scrutiny with any form of diagnostic testing. However accurate or sensitive a test may be, in the hands of an unskilled user results may not be appropriate.
Ovulation With many women leaving planning their families until later in life, the use of tests to provide an accurate check as to when they are ovulating would seem to be sensible.
Fertilisation of a released ovum is more likely during a 24-hour period after there has been a surge of luteinising hormone (LH) and this only occurs in regularly ovulating women once a month.
Ovulation testing kits provide an immunoassay technique that is very specific and sensitive. It involves the individual using urine to test for the LH surge usually around day 14 of the monthly cycle.
Pregnancy testing Again, the use of immunoassays measuring human chorionic gonadotrophin for pregnancy testing has made this type of testing easy and straightforward. Many GPs will rely on the patient's own diagnosis because of the ease of testing and as such this has reduced the number of routine tests performed in hospital laboratories. In 1997, it was estimated 70 per cent of independent pharmacies offered an in-pharmacy pregnancy testing service.
It is estimated there are more than a million undiagnosed diabetics in the UK, with a cost of 4.5 per cent of the NHS budget used to treat the resultant complication, so a screening programme for elevated blood glucose could provide some solution.
Blood glucose testing Both urine and glucose testing equipment is freely available for individuals to purchase but in most instances the use of urine testing is seen to be outdated.
A complication is when to test. Also one isolated elevated blood glucose level does not automatically lead to a diagnosis of diabetes. The process of testing involves the individual taking a sample of blood, placing it on a testing area, and then waiting for a reading. If patients themselves are testing there are a number of areas of error that may be introduced. Patients who are poor at getting blood from the puncture site may be inclined to squeeze the area, this leads to the leaching of tissue fluid, resulting in a falsely low blood glucose reading.
Also, where insufficient blood has been placed on the testing area a low reading may be obtained. Generally, the view would be with blood glucose testing the range of equipment available is generally accurate and precise, the experience of the tester is vital.
Coronary heart disease
A great concern in recent years is the level of CHD in the population resulting in heart attacks. This has led to the production of national service frameworks in key areas. In the UK, CHD affects 3.5 per cent of the UK adult population and causes 128,500 deaths a year.
Cholesterol testing By testing individuals for elevated levels of cholesterol in the blood its reduction after pharmacological and non-pharmacological treatment should be seen. The tests themselves are generally of a robust nature, but as with blood glucose testing, the skill of the tester, in this case the potential 'patient', is vital.
There is the issue of leaching of tissue fluid into a sample leading to a falsely low cholesterol reading together with a consideration of what is being measured. Whether the individual has fasted prior to taking a test then the expected level should be considered. Also, the use of an isolated test is considered to be useless. There has been much poor press regarding these tests6.
Blood pressure testing As with cholesterol testing, self-testing picks up some patients with previously undiagnosed hypertension. The well-known 'white coat syndrome' may be reduced by the correct use of such equipment.
Blood pressure testing equipment can be very easily purchased and with the correct training can give an accuracy of
+/-3mmHg. Again, the skill of the operator is paramount. If individuals are measuring their own blood pressure they are rarely aware that the position of their arm in relation to their heart can skew the result. An arm elevated from the heart gives a falsely low reading and an arm lowered relative to the heart a high reading. In patients where blood circulation is an issue, wrist or finger BP monitors should be discouraged as this again can lead to falsely low readings. The British Hypertension Society produce ratings for BP monitors.
How accurate is diagnostic testing and how much does it cost?
Test Accuracy* Price per test
Ovulation >99 per cent £4+
Pregnancy >99 per cent £6+
Blood glucose +/-10-15 per cent*** £6+
Cholesterol Variable*** £9+
Blood pressure +/-3mmHg Various **£30+
*when used in accordance with instructions
**depends on functions, some are self-inflating and may have a paper
readout and PC connectivity
***very variable depending on operator
Definitions in diagnostic testing
Testing of an individual is used to reduce clinical uncertainty allowing a doctor to diagnose or select appropriate treatment; here the objective of screening is to identify at-risk patients and raise awareness to a potential cause for concern.
By establishing the presence or absence of a particular disease in those patients presenting with, or often without, specific symptoms, treatment can be made earlier with the benefit of improving patient outcomes3,4,5.
When testing patients, the investigative test needs to be accurate and precise. The extent of agreement between the test value and the true value is termed the accuracy and is achieved by thorough quality control including regular calibration where appropriate.
Precision shows the results are reproducible when repeated utilising the same sample.
Specificity and sensitivity are measures of the performance of the diagnostic test. Specificity
relates to the probability that a healthy individual will have a negative result. The probability of a positive result in a diseased patient is termed sensitivity. The process for determining the test sensitivity and specificity involves comparing the test against a standard4.
Diagnostic testing maybe divided into a number of types:
· Diagnosis For instance, pregnancy testing.
· Monitoring of a chronic condition Many
long-term illnesses require regular monitoring of the therapeutic efficacy of treatment: therapeutic drug monitoring, blood glucose, peak flow, blood pressure.
· Preventive diagnostic testing or screening
Use of tests as a focus for promotion on a healthier lifestyle while alerting an individual to a cause for concern, this is primary prevention blood or urine glucose, cholesterol, carbon monoxide levels in smokers.