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Purpose of investigation: Difference between revisions
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* Screening for [[inborn errors of metabolism]] and [[developmental dysplasia of the hip]] in newborns | * Screening for [[inborn errors of metabolism]] and [[developmental dysplasia of the hip]] in newborns | ||
* [[Non-invasive prenatal test]] (NIPT) for trisomies during pregnancy | * [[Non-invasive prenatal test]] (NIPT) for trisomies during pregnancy | ||
Screening is indicated if the disorder has a high mortality or morbidity, and there is treatment available. | Screening is indicated if the disorder has a high mortality or morbidity, and there is treatment available. Like most things in life, screening is only viable if it can be proven that the advantages weigh up for the disadvantages. | ||
=== Screening tests === | === Screening tests === | ||
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=== Advantages of screening === | === Advantages of screening === | ||
Screening may have many advantages. The main advantage, and the intention behind screening, is that treatment can be started at an earlier time, before symptoms even appear, which usually improves the prognosis considerably, and may allow for curative treatment, which may not have been an option if the disorder was not diagnosed at the asymptomatic stage. | Screening may have many advantages. The main advantage, and the intention behind screening, is that treatment can be started at an earlier time, before symptoms even appear, which usually improves the prognosis considerably, and may allow for curative treatment, which may not have been an option if the disorder was not diagnosed at the asymptomatic stage. | ||
Most national screening programmes are backed up by evidence that they reduce mortality or morbidity. For example, breast cancer screening reduces risk of dying from breast cancer by approximately 20-30%.<ref>https://pubmed.ncbi.nlm.nih.gov/32326646/</ref> | |||
=== Disadvantages of screening === | === Disadvantages of screening === | ||
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South Korea started screening for thyroid cancer in the late 20th century. Up until relatively recently, research showed that, while the incidince increased significantly (more cases of thyroid cancer were discovered), the mortality remained the same. In other words, screening detected more cases and lead to more people being treated for cancer, which is a burden to both the healthcare system and the individual, but screening could not demonstrate a reduction in mortality, which is arguably one of the most important goals of screening. This shows that screening can lead to significant overdiagnosis.<ref><nowiki>https://www.nejm.org/doi/full/10.1056/NEJMp1409841</nowiki></ref> | South Korea started screening for thyroid cancer in the late 20th century. Up until relatively recently, research showed that, while the incidince increased significantly (more cases of thyroid cancer were discovered), the mortality remained the same. In other words, screening detected more cases and lead to more people being treated for cancer, which is a burden to both the healthcare system and the individual, but screening could not demonstrate a reduction in mortality, which is arguably one of the most important goals of screening. This shows that screening can lead to significant overdiagnosis.<ref><nowiki>https://www.nejm.org/doi/full/10.1056/NEJMp1409841</nowiki></ref> | ||
In Norway, it is estimated that for every 6 case of breast cancer that is discovered due to screening, 1 of those cases are overtreated, meaning that the cancer in that one case would not have needed treatment if it wasn't screened for. | |||
==== Disorders are often rare, which reduce the predictive value ==== | |||
As already established, pre-test probability influences the predictive value of a test. In asymptomatic people (the target population of screening tests), the pre-test probability is equal to the prevalence of the disorder. The disorder we screen for often have a very low prevalence; for example, breast cancer has a prevalence of 0,4%. | |||
Because the disorders we screen for is so rare, the positive predictive value decreases. | |||
==== A negative screening test does not guarantee disease-freedom ==== | |||
Even though most tests used to screen populations have high sensitivity and therefore a low rate of false negatives, false negatives still occur, giving many people the impression that they do not have a disorder when they do. This may lead to them not seeking healthcare for new symptoms which are actually due to a disorder, because they think the screening test has cleared them. | |||
==== Screening a population is expensive ==== | |||
For screening to be effective, it needs to screen many people, usually many thousands. This is expensive, money which could be used to research other fields of medicine. | |||
== Diagnosis == | == 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. There is no reason to perform an investigation if it doesn't help narrow doen the number of differential diagnoses. | 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. There is no reason to perform an investigation if it doesn't help narrow doen the number of differential diagnoses. | ||
=== Using test | === Using pre-test probability wisely === | ||
Because positive and negative predictive values depend on the | Because positive and negative predictive values depend on the pre-test probability, it's important to keep the it in mind. | ||
For example, a negative D-dimer has a high negative predictive value for VTE. However, negative predictive value decreases with increasing | For example, a negative D-dimer has a high negative predictive value for VTE. However, negative predictive value decreases with increasing pre-test probability. A person with a high pre-test probability (Wells score for PE of 7 or more) have a 40+% pre-test probability of PE. If the patient has a high pre-test probability, determined by their Wells score, the patient no longer has the same pre-test probability as the general population (which is low). The negative predictive value decreases, which makes a negative D-dimer unsuitable for ruling out PE in a person with high pre-test probability. | ||
== Follow-up and monitoring == | == Follow-up and monitoring == | ||