A visit to the hospital often means having a blood test. Emergency doctors and nurses, in particular, often bleed their patients, primarily to aid with making a diagnosis. The question is, how useful these blood tests really are.
In the emergency department (ED), as much as 70 per cent of all major clinical decisions may depend on blood test results. However, too many tests may be requested for any given patient, leading to more questions being raised than being answered. To reduce the time a person spends in the ED, blood tests are often requested at the front end, before he or she is formally seen by a doctor. This can lead to a so-called “shotgun” approach to blood testing, with more tests being requested than are actually necessary.
The large numbers of patients that attend, and the large numbers of blood tests requested, add to the risk of errors in both obtaining blood samples and in interpreting the results. The gold standard for requesting any blood tests is that they happen to be the right ones for the right patient at the right time. This is a standard that is frequently not met in a busy ED.
To start with, the wrong blood test may be requested. This may then lead to a false-positive result, leading to misdirected efforts which fail to address the patient’s specific needs. Many missed and delayed diagnoses in the ED occur simply because the correct blood tests have not been performed on a given patient.
The way in which the blood specimen is collected may itself be at fault. The wrong collection tube may be chosen. The tube may not be labelled or may have the wrong patient sticker affixed. Samples may thus get mixed up and mishandled. The blood specimen may be unsuitable for testing because it has clotted, haemolysed, or contains interfering substances. If the ED is busy, a repeat blood sample may then not be obtained and the test may be inadvertently omitted.
Errors in the collection of blood samples can be reduced by computerised order entry, automated procedures for blood collection , and computer-generated bar code identification of samples. Modern information and robotic technologies can come to the rescue by reducing the scope for human error.
Laboratory testing is the most reliable part of the process. Most procedures for analysis of blood samples are automated. All processes are subject to rigorous quality control, thereby ensuring that the laboratory is appropriately accredited. It follows that the results generated by the laboratory are mostly accurate and can be readily trusted. It is unusual for the result of an appropriately ordered test to be inaccurate.
Most problems arise when blood tests are interpreted by the doctor. For a start, it is surprising how often significant and important blood test results are overlooked-either not seen at all, or seen and forgotten, and thus not acted upon. One way this can be avoided is for laboratory staff to ring the requesting doctor with important results and have that person to repeat back the results on the phone.
Even when actually seen, the results may be misinterpreted. There may be confusion about the reference range of each individual substance or analyte that is being measured. The upper and lower limits of the reference range do not necessarily indicate the presence or risk of disease, but simply reduce uncertainty over the diagnosis. Very few biochemical tests clearly separate ‘normal’ from ‘abnormal’. For most tests, there is a range of values in which normal and abnormal overlap.
The perfect diagnostic test would always be positive in patients with a given disease and would always be negative in patients without that disease. This test does not exist, and this is the single biggest problem.
How good a diagnostic test is depends on how good it is at identifying people who have the disease (sensitivity) and those who do not have the disease (specificity). These measures of diagnostic accuracy are unfortunately not always clearly understood by doctors who interpret diagnostic tests.
To be truly well-informed, the patient needs to be aware of what blood tests have been requested and what the implications and limitations of the results are. Good dialogue with the doctor and shared understanding most certainly helps reduce errors in this area of health care.
Ashis Banerjee (ex-NHS)