Diagnostic reasoning is the structured thought process a clinician uses to determine a patient’s diagnosis, combining scientific knowledge with clinical experience. The process is iterative, involving the continuous gathering and integration of information to arrive at a conclusion. This article demystifies the process through a step-by-step clinical example.
The Initial Clinical Encounter
The diagnostic process begins with the initial clinical encounter, where the goal is to collect detailed information from the patient, known as the History of Present Illness (HPI). For example, consider a 22-year-old male who presents to an emergency department with abdominal pain. The clinician characterizes this pain through structured questions to build a comprehensive picture of the symptoms.
Questions explore the onset, location, and duration of the pain. The patient might report that the pain began about six hours ago as a dull ache around his belly button. Over the past few hours, the pain has moved to the lower right side of his abdomen and has become sharper. The pain is described as steady and is made worse by movement, such as walking or coughing. This migration of pain from the periumbilical region to the right lower quadrant is a significant clue.
Further inquiry focuses on alleviating or aggravating factors and any associated symptoms. The patient may state that nothing seems to make the pain better. He also reports experiencing nausea, a loss of appetite, and a low-grade fever that started around the same time as the pain. Vomiting that begins after the onset of pain is a common finding, whereas vomiting that precedes pain might suggest a different condition like an intestinal obstruction.
Developing a Differential Diagnosis
With the initial history gathered, the clinician develops a differential diagnosis, which is a list of potential conditions that could explain the symptoms. Each potential diagnosis is a hypothesis to be tested. For the 22-year-old male with right lower quadrant abdominal pain, several possibilities are considered based on this initial information.
The most likely diagnosis given the presentation is acute appendicitis. This is included because the migration of pain from the umbilicus to the right lower quadrant, coupled with nausea, anorexia, and fever, is a textbook presentation of an inflamed appendix.
Another possibility is a kidney stone, medically known as nephrolithiasis. This is considered because a stone passing down the right ureter can cause pain that radiates to the lower abdomen. However, this pain is often described as colicky—coming in waves—and may be associated with blood in the urine, which was not reported by this patient. Gastroenteritis, or a stomach bug, is also on the list due to nausea and abdominal discomfort, though its pain is more diffuse and often accompanied by significant diarrhea, which this patient does not have.
Other less common considerations include inflammatory bowel disease, such as Crohn’s disease, which can cause right-sided abdominal pain, though it is usually chronic. In a different patient, such as a female, the differential would be broader, including gynecological issues like a ruptured ovarian cyst or an ectopic pregnancy. These conditions are weighed against the patient’s story, with appendicitis emerging as the leading hypothesis.
Refining the Diagnosis Through Investigation
After formulating a differential diagnosis, the clinician gathers objective data through a physical examination and diagnostic tests to test the hypotheses. The physical exam begins with checking vital signs, which might confirm the patient’s reported low-grade fever of around 38°C (100.5°F).
The abdominal examination involves palpating the abdomen for tenderness. In this case, tenderness would likely be found in the right lower quadrant over a location known as McBurney’s point. The presence of rebound tenderness (worsening pain when pressure is released), rigidity, and guarding are indicators of peritoneal inflammation, increasing the likelihood of appendicitis.
Based on these findings, laboratory tests are ordered. A complete blood count (CBC) is obtained to check for signs of infection; a high white blood cell count, particularly an elevation in neutrophils, would support the diagnosis of appendicitis. A urinalysis is also performed to rule out a kidney stone or a urinary tract infection. If the urinalysis is clear, it makes a kidney-related issue less probable.
To visualize the appendix and confirm the diagnosis, an imaging study is ordered. An abdominal computed tomography (CT) scan is highly accurate for diagnosing appendicitis and can show an enlarged, inflamed appendix or complications like a small abscess. For certain populations, like children or pregnant women, an ultrasound may be used as the initial imaging test to avoid radiation exposure.
Arriving at the Final Diagnosis
The final step is to synthesize all collected information—history, physical exam, lab results, and imaging—to arrive at a diagnosis. The clinician reviews the evidence to determine which condition it most strongly supports. For the 22-year-old male, the information points to a clear conclusion.
The patient’s history of migrating pain, nausea, and anorexia suggested appendicitis. The physical exam provided objective evidence with localized tenderness at McBurney’s point and signs of peritoneal irritation. Laboratory results supported this with an elevated white blood cell count of 12 x 10⁹/L with 85% neutrophils, indicating an active infection.
The CT scan showed a swollen, inflamed appendix, confirming the diagnosis of acute appendicitis. This combination of findings also made other diagnoses less likely. The normal urinalysis argued against a kidney stone, while the localized pain and lack of diarrhea made gastroenteritis improbable.
With the diagnosis of acute appendicitis confirmed, the clinician communicates the findings to the patient and initiates treatment. This involves consulting a surgeon to perform an appendectomy, the surgical removal of the inflamed appendix. This is the standard treatment to prevent rupture and other complications.