What a diagnosis claims
A diagnosis identifies what is thought to be wrong with a patient, but its label may describe a cause, a pattern of findings, a level of certainty or the evidence used to reach it.
Diagnosis as a process and a label
A diagnosis needs to be used accurately because the patient's care depends on it. It answers what is wrong, alongside separate questions about cause, treatment and prognosis. Sometimes the diagnosis partly answers those other questions: diagnosing influenza implies infection with an influenza virus and affects treatment and predictions about the illness.
Diagnosis is both the process of identifying a condition and the label applied at its conclusion. A differential diagnosis can mean the process of choosing between conditions with similar clinical presentations, or the list of possible explanations being considered. Each item on that list is a diagnostic differential.
A diagnosis expresses a belief that needs justification, such as a characteristic sign or symptom, imaging, laboratory testing or a formal case definition. A pathognomonic finding is distinctively characteristic of a condition, while a case definition specifies criteria that must be met. Meeting the apparent criteria does not guarantee that the diagnosis is correct.
Diagnostic heuristics include considering common conditions first, while remembering that rare diseases occur. Seeking one explanation for all findings follows Occam's razor, but must be balanced against Hickam's dictum that a patient can have several diseases.
What different labels describe
Diagnostic labels classify illnesses and group patients, making clinical studies possible. Some describe what is believed about the condition, while others describe how that belief was justified. The following terms distinguish the setting, evidence, certainty or purpose of a diagnosis.
| Term | Meaning |
|---|---|
| Admitting diagnosis | The diagnosis reported by the clinician deciding on hospitalisation, recorded for administrative purposes and possibly tentative before examination and testing. |
| Biologic diagnosis | A diagnosis based on testing a patient's specimen in a laboratory animal, egg or cell culture, for example to isolate a virus. |
| Clinical diagnosis | A diagnosis based on symptoms, examination signs and history; in comparison with autopsy findings, it can include all information available during life, including laboratory and imaging results. |
| Cytologic diagnosis | A diagnosis based on examination of exfoliated or shed cells. |
| Definitive diagnosis | A final diagnosis based on confirmatory testing. |
| Diagnosis of exclusion | A diagnosis reached by eliminating all other known differentials. |
| Diagnosis ex juvantibus | A diagnosis based on the response to treatment, such as antibiotic therapy. |
| Direct diagnosis | A diagnosis based on structural lesions or pathognomonic signs or symptoms. |
| Discharge diagnosis | The diagnosis recorded when the patient leaves hospital. |
| Etiologic diagnosis | A diagnosis implying the cause of illness, or the process of identifying that cause, such as isolating a bacterial strain. |
| Faux diagnosis | A false diagnosis made for an administrative purpose. |
| Laboratory diagnosis | A diagnosis based on laboratory tests or examination of specimens such as tissue or body fluids. |
| Niveau diagnosis | Localisation of the exact level of a lesion, such as in the spinal cord. |
| Pathologic diagnosis | A diagnosis based on observed structural lesions. |
| Physical diagnosis | A diagnosis based on inspection, palpation, percussion and auscultation of the body. |
| Presumptive diagnosis | A diagnosis based on the known probability of a condition, such as a diagnosis made during life when confirmation is possible only at autopsy. |
| Provocative diagnosis | A diagnosis based on a provocative test, such as exposure to an allergen to confirm an allergy. |
| Radiologic diagnosis | A diagnosis based on imaging, such as x-rays, computed tomography or magnetic resonance imaging. |
| Serum diagnosis | A diagnosis based on testing serum, the fluid remaining after a blood sample coagulates. |
| Syndromic diagnosis | Recognition of a pattern of signs and symptoms that may not always have the same cause. |
| Wastebasket diagnosis | A vague and possibly meaningless label for a condition that resists classification. |
| Working diagnosis | The differential or set of differentials considered most likely to be correct. |
Symptoms are subjective experiences that only the patient can observe, whereas clinical signs can be observed by the examiner. A diagnosis can change from syndromic to etiologic as its cause becomes known, as happened with several classic childhood exanthems.
Diagnostic errors
A misdiagnosis assigns a condition the patient does not have, for example when diseases have similar signs and symptoms or a diagnostic test produces a false-positive result. A missed diagnosis fails to identify the correct condition, for example after a false-negative test or premature closure of the differential that leaves the correct possibility out of consideration.
Underdiagnosis means that a condition is missed in a significant proportion of cases. Poor access to care, limited awareness among the public or clinicians, and difficulty making the diagnosis can contribute. Diagnostic labels can also be applied to cases too mild to warrant medical attention. Inappropriately widening criteria and aggressively promoting awareness to expand markets for diagnostics or treatments is called disease mongering.
Diagnostic errors can lead to poor treatment decisions and outcomes for individuals, and poor public health or misallocated resources across a population.
Boundaries and diagnostic meaning
Even a diagnosis with two conceptual categories can be difficult to apply to a particular person, and a yes-or-no question may remain unanswered. A single clear criterion can still lead to misclassification if it is hard to document. For measurement-based diagnoses, the accuracy of the measurement, the meaning of the scale and the placement of category boundaries all need consideration. A person close to a weight-category boundary may cross it and back within one day.
Different labels can make differences of degree appear to be differences of kind. A spectrum disorder describes conditions that look different but are believed to represent the same underlying problem at different levels of severity.
A label can also misrepresent the cause of a person's difficulties. In 1851, Samuel A. Cartwright labelled attempts by enslaved people to escape as a supposed mental illness, “drapetomania”. Writers should be alert to bias towards either biological or psychosocial explanations, including labels that attribute physical symptoms to psychological causes or treat social and educational problems as medical ones.
Questions for writing about a diagnosis
Five questions help establish what a diagnostic label means.
- Does it assert a cause, or recognise a familiar pattern of signs and symptoms?
- How is it justified: by physical examination, imaging or laboratory testing?
- How meaningful are the criteria, and how reliably are they applied?
- How severe must a case be to qualify, and how were boundaries between normal and abnormal values set?
- Does the label say something true about the patient, or send a misleading message that undermines the patient's interests?