Purpose of investigation
When one orders a test (laboratory, imaging, or otherwise), it's important to have a predetermined purpose for the test and not just ordering tests willy-nilly. Generally, investigations can be used for:
- Screening (healthy or not healthy)
- Lab diagnosis
- Follow-up/monitoring
- Monitoring progression/response to treatment
- Evaluation of prognosis
- Information regarding the likely outcome of the disease
Value to the patient
An investigation should have some form of value for the patient (except if done in a research setting or for epidemiological purpose, in which case the investigation has value for other patients). Common examples to consider:
- A frail elderly patient living in an institution (like a nursing home) has a positive faecal occult blood test. The physician considers to refer them to colonoscopy, but stops to consider: how will a colonoscopy be of value to the patient? In their current state, even if the colonoscopy would show colorectal cancer, the patient would not be a candidate for any anticancer surgery or chemotherapy, so even if a diagnosis is made, nothing will change for the patient (except the stress of knowing they have cancer). In addition, colonoscopy is an invasive investigation which requires strict patient preparation, which can be difficult for a frail elderly to perform or even survive.
- If they have colorectal cancer and it grows and eventually causes intestinal obstruction, colonoscopy may be indicated for stenting the bowel, in which case the colonoscopy would have value for the patient as palliative therapy
- A young male has had back pain for a few days. The pain is not severe, and there are no red flags for cauda equina syndrome. He wants an MRI to know what's going on, but the physician stops to consider: how will an MRI be of value to the patient? The patient may or may not have a herniated disc, but even if they do, there are no indications for surgery for the herniation. As such, whether the MRI shows a herniated disc or not, the management will be the same (no surgery, only physical therapy and pain relief), and the MRI is therefore of no value to the patient (and it is resource-intensive)
- On the other hand, if the patient has debilitating pain or there are red flags present, he may have cauda equina syndrome, in which case surgery is indicated. In this case, MRI has value: it determines whether they need surgery or not
- A middle aged woman has symptoms of an upper respiratory tract infection. After taking the anamnesis and physical examination, you're certain that it's a viral infection. You reflexively want to order a CRP or leukocyte count, but you stop to consider: will the laboratory investigation be of value to the patient? You know that viral URTIs only cause mildly elevated inflammatory parametres, so that's likely what you'll find anyway. And even if the CRP is higher than you expect, you're certain enough that this is not a bacterial infection, so you won't be administering antibiotics anyway. So even after making the investigation, you'll most likely not be changing your management of this patient; managing their symptoms and encouraging rest, without antibiotics
- On the other hand, if the patient has symptoms which make it difficult to distinguish between viral and bacterial infection clinically, a laboratory investingation is merited, as it provides additional information which can aid in the diagnosis and therefore the treatment in this case
- Consider that many laboratories can analyse a pharyngeal swab for specific airway viruses. Would making such an investigation in this case change the management of the patient? In most cases no, as there is no specific treatment for most airway viruses anyway.
Screening
Screening refers to using an investigation to detect a disease which has not yet caused symptoms, so-called subclinical disease. Examples include:
- Regular mammography or cervical cytology in women
- Used to detect early or precursor breast cancer or precursor stages for cervical cancer, respectively
- Faecal occult blood test in middle-aged/elderly
- Used to detect early colorectal cancer or bleeding colon polyps
- Screening for inborn errors of metabolism in newborns
A screening test is most useful when it has high sensitivity. It would be great if a screening test also had high specificity, but there is often a trade-off between sensitivity and specificity. Therefore, most screening tests have low specificity.
Because screening test have low specificity, a positive screening test must always be confirmed by a more specific test.
Diagnosis
When an investigation is ordered for diagnosis, one should already have a list of differential diagnoses before ordering the test, and the test should be able to narrow down the list of differential diagnoses. It's important to consider the test's specificity, sensitivity, positive predictive value, and negative predictive value in this.
Follow-up and monitoring
Follow-up and monitoring of a disorder is also a common use of laboratory and imaging investigations. Examples include:
- CRP or leukocyte count following antibiotic prescription to evaluate the treatment response
- Yearly brain MRI following removal of a meningeoma, to try and catch a recurrence early, before it causes symptoms
However, it's important to keep in mind the requirement of value for the patient. In the first scenario, there is value as a lack of normalisation of inflammatory parametres in a patient treated with antibiotic for a bacterial infection may be a sign that the antibiotic is ineffective, which may require administration of a different antibiotic. In the second scenario, there is also value, as the earlier one can catch recurrence of a meningeoma, the better the prognosis after treatment. However, if the patient is in a condition where they will not receive treatment anyway (for example, if they are terminal), monitoring is not of value to the patient.