Laboratory Blood Tests - FAQs

General Health

Medichecks' Clinical Pathologist Dr Peter Prinsloo answers some common questions about laboratory testing.


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1. How does the laboratory ensure that its results are accurate?

Medichecks' samples are analysed by two laboratories, both of which are accredited and inspected for overall quality management by UKAS (The United Kingdom Accreditation Service). These laboratories strive to ensure that the right results are always obtained and this is achieved by the process of internal quality control (IQC) and external quality assessments (EQA). Furthermore they participate in regular quality audits, clinical governance and accreditation inspections.

Practically, IQC is performed on a daily basis and sometimes more than once per day depending on the test that is being done. Samples containing quality control material (very similar to blood but with exact known amounts of test material in the sample) are measured on the analysers. The results produced by the analyser should fall within very strict pre-set limits that agree with the exact amount in the test sample at different levels i.e. low, medium and high levels. A patient’s blood sample will only be analysed when the quality material results have passed all the control criteria.

All accredited laboratories are required to participate in an external quality assurance scheme (EQAS) for all the tests that they provide. These schemes are national schemes i.e. NEQAS (National EQA Scheme) or WEQAS (Wales EQA Scheme).  The EQA schemes send out samples to all laboratories either fortnightly or monthly, depending on the tests being checked. These samples contain known amounts of the specific “test” that is being assessed, however the laboratories being assessed do not have knowledge of the true “target” value and have to measure these samples and report the results to the EQA scheme.  If these results are not within certain acceptable limits the laboratory will receive a warning or a “poor performance letter” and if the problem is not rectified, the laboratory may decide to stop offering the test or be required to do so by UKAS.

Example of part of an EQA report showing the different group of analysers and the spread of results. The arrowhead indicates where the result of the laboratory being tested falls and the blue highlights the results produced by all laboratories that use that specific method. This report indicates the expected variability between laboratories and also the reason for differing reference ranges (“normal ranges”) between laboratories for TSH. 


 
In the following example there is a clear difference in the values obtained by different analysers and therefore usually cortisol as an analyte will have two very different reference (“normal”) ranges for cortisol depending on the type of analyser that the laboratory use.

 

2. Why did the blood results of the tests I had done through Medichecks differ from the tests my doctor performed?

There are several factors that affect blood results which can be summarised under three headings.

a) Biological variation (intra-individual variation):

In any individual there are biological (diurnal) cycles that may affect blood results i.e. cortisol is at the highest at around 8–9 am in the morning and very low at midnight. Similarly testosterone levels are highest at 8-9 am decreasing throughout the day so the results may vary depending on the time of day a sample is collected.

b) Pre-analytical variation:

These are variations or changes that may occur before the samples are tested i.e. delay in reaching the laboratory, samples being left in a cold environment during transport, volume of blood collected, whether the tourniquet was left on or removed before collection of the sample etc. These changes are mostly small and in majority of cases will not impact on treatment but may cause differences in the result i.e. sodium of 139 and a sodium of 143 on the same person collected a short time apart or even at the same time. Occasionally results may be significantly altered by cold, heat, time delay etc. Specifically potassium results can change from collection to analysis and therefore Medichecks do not offer these tests as a matter of routine.

c) Analytical imprecision:

All laboratory analysers show some variation in a similar way that a size 10 dress from different manufacturers vary in size, or the speedometers in two cars may differ at the same true speed. These laboratory variations are termed analytical imprecision and every laboratory has set criteria for what is acceptable and what is not. If an internal quality control sample with a true testosterone level of 10 is measured on the same analyser for 20 times, on the same afternoon, the results may vary between 9.5 and 10.5 (therefore giving values of between 5% above and 5% below the true value). These variations are called the “coefficient of variation” (CV) of an assay and are a measure of the precision of the assay.

Different laboratories often have different analysers and every major manufacturer derives a “normal/reference range” for each test. Therefore the blood test from your GP may have a different reference range to the Medichecks result and the blood result would also be different.   
 
3. Are NHS tests more accurate?

As discussed above all laboratories are subject to the same strict rules and quality control procedures and therefore NHS tests are not more accurate. Very often NHS laboratories would use the same analysers and quality control material as the Medichecks laboratories.
 

4. What is the process of accreditation?

The accreditation of medical laboratories in the UK is managed by UKAS (United Kingdom Accreditation Service) which is the sole national accreditation body for the UK and it is operated under supervision of the government. UKAS accreditation ensures:

a) Conformity between laboratories
b) The laboratory staff are competent to carry out your diagnostic tests
c) The equipment that is being used is fit for purpose
d) You are provided with a quality lead service with patient safety at its core

UKAS inspect the accredited laboratories at regular intervals and any non-conformances found are raised and the laboratory is expected to rectify the issues and provide evidence that this has been done within three months. Between the main inspections the laboratories are also subject to surveillance visits to ensure the quality management is up to date.

5. What is an acceptable difference between one set of blood test results with another?

All blood tests show a degree of variability even when the same sample is tested on the same analyser on the same day on consecutive times. This is called the analytical variation of a test and is calculated by determining a number called the 'coefficient of variation" (CV). The CV for every test also varies according to the level of the test being performed. In the example below sodium has a CV of 1.5% at a level of 140 which is an acceptable level of variation. (At a sodium level of 110 the CV may be 2% and at 170 also 2%)
Therefore at a true sodium level of 140 the measured sodium may be anywhere between 137.9 and 142.1 (true result plus or minus 1.5%).  In statistical terms these calculations may become very complicated and has therefore been simplified. Therefore on the same day one laboratory may report the sodium on the same sample as 142.1 and another laboratory may report 137.5. Although there is a difference in the results, this difference is not of any clinical significance. 
For some tests such as hormone tests the variation is usually higher with CVs between 3 - 10%.   

6. Can I trust a finger-prick blood test? Surely a venous sample is more accurate?

There is a misconception that venous samples are more accurate then finger-prick blood testing but this is not always true. Finger-prick samples have been proven as accurate as venous samples provided that the samples have been collected correctly and an adequate sample volume has been obtained. There are certain tests where a full venous sample collection is required and where a finger-prick sample would not give an accurate and true result.

7. What else can affect my blood test results?

There are numerous (literally thousands) of factors that may have an effect on blood results and the laboratory has access to extensive lists of all these "interferences".  Every blood test is affected differently by different factors i.e.:

a) Following a short burst of exercise or unusual activity such as decorating, muscle enzymes (CK or AST) may be increased for a few days up to a week. Similarly the CK concentration may be up to 20 to a 100 times higher than the normal after something as simple as a cramp in the calf or in a person suffering from flu.

b) Certain liver enzymes i.e. GGT may be permanently high in patients on anti-epilepsy medication but GGT may also show a temporary increase for a few days after having a couple of alcoholic drinks.

c) During, and after acute illness, thyroid function tests may decrease or increase reflecting the body's changing need for these hormones. Therefore it is wise to wait up to 6 weeks after any acute illness (chest infection, kidney infection etc) before testing the thyroid function.

d) Numerous drugs and food may interfere or have an effect on many different tests. The laboratory would detect any unusual interference and often this would require retesting. 

Having knowledge of the medication and lifestyle history is very helpful when assessing any possible interference.   

8. What should I do if I get an unexpected result?

It is usual practice in the NHS and also in Private Practice to repeat any blood test that has yielded a very unusual or unexpected result. Apart from quality procedures to ensure accurate results the laboratories have additional mechanisms to ensure that the correct result is reported including technical validation and clinical validation. Technical validation is where the entire process is technically assessed and unexpectedly abnormal results are then repeated by a different method, or on dilution or using blocking agents and antibodies. Clinical validation is the process whereby clinical detail is checked against the results to ensure the results would fit in with the clinical detail. Despite these two processes, there are times where due to interferences within the body such as medication, disease and diet, very unusual results are produced. In these cases it may be helpful to have a repeat test and avoid factors that may influence the results i.e. abstain from exercise before the test, avoid certain drugs and foodstuff such as bananas etc.

9. What does it mean if my result is outside the normal range?

The "normal" range for any given population is usually the range where 95% of that population will fall within. This means that 2.5% of that population will be above the normal range and 2.5% will be below the normal range. At the edges of the normal range there are areas of overlap where healthy individuals will have results outside the "normal" and unwell individuals will have results inside the normal range. 

Very few tests are done in isolation i.e. thyroid function tests usually include between 2 to 4 tests or liver function tests include between 4 to 7 tests and these tests are all interpreted in context of the entire panel of tests. An interpretative comment is added by the doctors at Medichecks to ensure a single result outside the "normal" range is not interpreted as a significantly abnormal result, as mostly it is not of any clinical significance when interpreted in the context of the full result. If an abnormal result is cause for concern advice will be given on additional blood tests or seeing your GP to discuss further review. 

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