There are many reasons why we request tests, in medicine. One imaginary patient’s journey picks up a number of them.
Take a patient who presents with a painless lump on their arm, who’s tired and a bit pale & washed out. You might send a series of blood tests, including a full blood count to diagnose anaemia. You may also request an ultrasound of the lump, which may show an ugly mass with features consistent with sarcoma. Your friendly local plastic sarcoma surgeon might do a biopsy for you after an MRI, and the histopathologists confirm it’s a rhabdomyosarcoma.
All these tests are aimed at making a diagnosis: to clarify if the patient in front of us has, or does not have, the condition.
The oncologist who then takes up the patient’s care will move to undertake a series of further investigations;
bone or PET scanning, marrow biopsies and CT chest. These scans are not to make the diagnosis – that’s known – but to locate the disease, explore it’s extent and ultimately come up with a risk-stratification which may inform prognosis, affect choice of treatment, or both.
Should the disease be metastatic, treatment will include anthracyclines. This group of chemotherapy agents will mean the patient is subject to repeated echocardiography, looking for early signs of drug-induced cardiomyopathy. Here, the test is used to monitor function to assess for the development of a condition.
In those who present very young with a distinct subtype of rhabdomyosarcoma may now also be offered gene testing for a cancer predisposition syndrome: in this situation, the patient is being screened to highlight a potentially relevant issue for their future.
The ways we look to appraise a test vary according to its use: a test to diagnose may be appraised for its accuracy; a test to stratify risk, by its ability to predict outcomes and the separation of the risk groups; a monitoring test by its ability to change management and prevent adverse consequences developing; and a screening test by both the accuracy of the test itself and the interventions that can ameliorate anything discovered.
(There are then also those tests we do to make ourselves feel better. Like the chest x-ray when we’re going to give a short course of amoxicillin regardless of what the picture shows …)