What is an acceptable difference between one set of blood test results and another?
All blood tests show a degree of variability even when the same sample is tested on the same analyser on the same day at 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.
For example, 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 it may also be 2%). Therefore, at a true sodium level of 140 the measured sodium may be anywhere between 137.9 and 142.1 – the true result plus or minus 1.5%. On the same day, one laboratory may report sodium on a sample as 142.1 and another laboratory may report the same sample as 137.5 – although there is a difference in the results, it is not considered to be clinically significant.
For some tests such as hormone tests, the variation is usually higher with CVs commonly between 3 - 10%.
Why did my blood test results from Medichecks differ from those my doctor performed?
There are several factors that affect blood test results which can be summarised under the following headings.
Biological variation (intra-individual variation)
In any individual, there are biological (diurnal) cycles that may affect blood results. For example, cortisol levels in the blood are 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.
These are variations or changes that may occur before a sample is tested – and may be dependent on the delay in reaching the laboratory, being left in a hot or cold environment during transport or the volume of blood collected. These changes are mostly small and in the majority of cases will not impact on treatment but may cause differences in the results. Occasionally results may be significantly altered by cold, heat, time delay or other factors.
All laboratory analysers show some variation in a similar way that a size 10 dress from different manufacturers varies 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.
Are NHS tests more accurate than private blood tests?
All accredited laboratories are subject to the same strict rules and quality control procedures as one another and therefore NHS blood tests are not inherently more accurate than private tests, or vice versa. Very often NHS laboratories use the same analysers and are part of the same quality assurance schemes as the laboratories used by Medichecks.
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 to be 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.
How do I know whether the laboratory testing my results is UKAS accredited?
All laboratories that are accredited by UKAS are listed on their website (www.ukas.com). The principal laboratories that Medichecks uses to analyse samples (County Pathology and The Doctors Laboratory) are both accredited by UKAS.
Check out our blood testing guide for more information