The validity and reliability of test results are dependent on a number of variables.
Preanalytical variables include physiological factors, patient preparation, specimen collection and transport.
Analytical variables include the precision and accuracy of the test method and factors which may interfere with a particular assay e.g. lipaemia, in vitro haemolysis, and medications.
Post-analytical variables include data entry and calculations by laboratory staff, result validation, interpretation of the result, data transfer and the method used to report the results (electronic, paper or telephone).
How does the Laboratory Control Variables?
The laboratory controls the impact of these variables, as far as possible, by two processes:
On each occasion that patient samples are tested, the laboratory also tests 'standards' with known concentrations of analytes or cells, of known reactivity in the test system. These controls need to be comparable to the test samples and are wherever possible “matrix matched” i.e. Urine based control material is used to control urine assays, serum based controls are used to control serum assays and whole blood controls are used to control whole blood assays.
All Gribbles laboratories participate in external quality assurance programs in which the results from each laboratory are compared to the results obtained by a group of laboratories. Quality assurance also requires laboratories to attend to pre-analytical and post-analytical variables although many of these factors may not be under their direct control. Please refer to our Quality Management Systems page to read more about this.
Variables outside the Laboratory’s Control
Quality control and quality assurance help maintain both the accuracy and the consistency of laboratory results but absolute accuracy is not technically possible. Variables cannot be entirely avoided and the interpretation of any result must take these factors into consideration.
Selection of the Test(s)
Discretionary testing is the selection of a single test, or a small number of tests, on the basis of the clinical findings.
Profile testing is the ordering of tests as 'screening tests' and may involve a 'battery' of tests.
In a normal individual, the greater the number of tests performed, the greater is the chance of finding an abnormal result. For example, if a 20 test profile is performed on a completely normal individual, there is a 64% chance that at least one test will be 'abnormal'. These 'abnormal' tests are usually of no clinical significance, but may need to be followed-up by repeat or additional testing which may lead to patient anxiety and so it may be necessary to explain this to the patient.
The laboratory result is dependent on the quality of the specimen which it receives. For example:
An inadequate biopsy or a poor cervical smear results in an incomplete and possibly inaccurate opinion.
An improperly collected urine specimen may result in a pathogen being masked by contamination with commensals.
An incorrect volume of blood in a citrate tube will result in incorrect results of coagulation tests.
Refrigeration of blood will result in an elevated potassium level.
The Request Form
Provision of appropriate clinical information is essential if the pathology laboratory is to assess the results and their likely significance. Any difficulty in obtaining the specimen should also be noted on the request form as this may affect the test result.
Quantitative Test Results & the Reference Interval
Reference intervals represent the test results which would be obtained in the normal population and are based on the results obtained (mean ± 2 standard deviations) on a series of normal 'healthy' individuals.
The levels quoted thus lie between the 2.5 and 97.5 centiles for the group from which they were derived.
Age, gender, race and test methodology are important variables so the reference intervals quoted in the literature may not be generally applicable.
Wherever possible laboratories establish their own reference intervals but this is not always feasible.
As the reference interval represents the 2.5 to 97.5 centiles, inevitably 5% of entirely normal people will have test results outside the reference interval. Minor variations should thus be interpreted with caution.
The Sensitivity & Specificity of a Qualitative Test
The ability of a test to discriminate between normal and abnormal individuals is described by its sensitivity and specificity.
Test sensitivity is defined as the percentage of people with a specific disease who have an abnormal test result.
Test specificity is defined as the percentage of people without the disease who have a normal result.
Generally, the balance of sensitivity and specificity is a major consideration in the choice of a test methodology. This balance must consider the ability of the test not to have false negatives (miss diseased individuals) and the ability not to produce false positives. Most screening tests employed will be biased towards reducing false negatives which may lead to approximately 2.5% of unaffected (normal) individuals having false positive result.