Appendicitis is a common and urgent condition involving the inflammation of the vermiform appendix, a small pouch attached to the large intestine. Diagnosing this condition relies on combining a patient’s symptoms and physical examination findings with supporting evidence from laboratory tests. Blood tests are routinely performed to quickly identify signs of infection and systemic inflammatory response. While not providing a definitive diagnosis, these results offer objective data that helps physicians determine the severity and likelihood of appendicitis.
The Primary Test: White Blood Cell Count
The initial blood test performed for suspected appendicitis is the Complete Blood Count (CBC). Of particular interest is the White Blood Cell (WBC) count, which typically becomes elevated in response to infection or inflammation. This elevation, known as leukocytosis, suggests the body is mounting an immune response to the localized problem. While the normal range for WBCs is generally between 4,500 and 11,000 cells per microliter, most patients with acute appendicitis present with a count exceeding 10,000 cells per microliter.
An elevated WBC count is present in approximately two-thirds of patients who have acute appendicitis. However, the specific types of cells must also be examined, not just the total number. Neutrophils are the most numerous type of white blood cell and are the first responders to bacterial infections, such as those that occur when the appendix becomes obstructed.
A specific finding often associated with appendicitis is a “left shift,” meaning there is an abnormally high proportion of immature neutrophils, also called band cells, in the blood. This shift occurs because the bone marrow rapidly releases less mature neutrophils to combat the escalating bacterial infection. The presence of both a high total WBC count and this left shift is highly suggestive of acute bacterial inflammation. A significantly higher WBC count, specifically \(\geq\)17,000 cells per microliter, is sometimes associated with more complicated cases, such as those that have progressed to gangrene or perforation.
Secondary Blood Markers of Systemic Inflammation
Beyond the CBC, other blood markers confirm the presence of broader systemic inflammation supporting the suspected diagnosis. The most commonly used secondary marker is C-Reactive Protein (CRP), a protein synthesized by the liver in response to pro-inflammatory molecules. CRP levels rise rapidly within 6 to 12 hours of inflammation onset, making it a sensitive indicator of an ongoing inflammatory process.
In the context of appendicitis, high CRP levels, often greater than 1 mg/dL, are frequently observed. If a patient has had symptoms for more than 24 hours, a normal CRP level has a high negative predictive value, suggesting appendicitis is unlikely. High CRP levels, especially when combined with leukocytosis and neutrophilia, may indicate a more severe or advanced stage of the disease, such as gangrenous appendicitis.
A less specific indicator of generalized inflammation is the Erythrocyte Sedimentation Rate (ESR). This test measures the rate at which red blood cells settle in a tube of blood over an hour, which speeds up when inflammatory proteins are present. While an elevated ESR suggests inflammation, it rises more slowly than CRP and is not specific to appendicitis, as many other conditions can raise it. Therefore, CRP and the WBC count are considered more direct and useful markers for evaluating suspected appendicitis.
Interpreting Results in the Diagnostic Process
Medical professionals do not rely on blood test results alone to diagnose appendicitis; they integrate them with a patient’s clinical presentation, physical examination, and medical history. The results function as one piece of evidence, primarily confirming the presence and extent of infection and inflammation. For instance, a patient presenting with classic right lower quadrant pain and a high WBC count with a left shift is considered to have a high probability of appendicitis.
To standardize this synthesis of information, healthcare providers often use diagnostic scoring systems like the Alvarado score. This system assigns numerical points to symptoms, physical signs, and laboratory findings. Blood tests contribute up to three points: two points for leukocytosis (WBC > 10,000/mm³) and one point for a left shift (neutrophilia). A score of 7 or higher on the Alvarado scale strongly suggests appendicitis and often leads to surgical intervention.
The limitations of blood tests are clear: a high WBC count can be caused by many other conditions, and up to one-third of patients with confirmed appendicitis may initially have a normal WBC count. Imaging is often necessary, especially for patients who fall into a diagnostic “gray zone,” such as an Alvarado score between 5 and 6. Imaging techniques, typically a CT scan or ultrasound, visualize the appendix directly and provide definitive confirmation or exclusion of the diagnosis. Normal blood test results can also help rule out a severe bacterial infection, prompting the physician to consider other causes of abdominal pain, such as a urinary tract infection or kidney stones.