The Truelove and Witts index for ulcerative colitis severity calculator assesses the severity of ulcerative colitis based on six user-provided clinical parameters. These parameters include the number of bowel movements per day, the presence and amount of blood in stool, body temperature, pulse rate, hemoglobin levels, and the erythrocyte sedimentation rate (ESR).

The user selects from predefined options for each parameter:

1. Bowel movements per day: "5", "<= 4", ">= 6".
2. Blood in stool: "None or small amounts of macroscopic blood", "Between visible and small amount of blood", "Visible macroscopic blood".
3. Temperature >= 37.8 degrees C (100.04 degrees F): "No", "Yes".
4. Pulse > 90 bpm: "No", "Yes".
5. Hemoglobin <= 105 g/L: "No", "Yes".
6. Erythrocyte sedimentation rate, mm/hour: "<= 30", "> 30".

The calculator interprets these inputs to classify the ulcerative colitis severity as mild, moderate, or severe according to specific criteria.

The decision process involves evaluating each input combination:

1. Mild ulcerative colitis is determined if:
   - Bowel movements are exactly 5.
   - Blood in stool is none or small amounts of macroscopic blood.
   - Temperature is below 37.8 degrees C.
   - Pulse is 90 bpm or less.
   - Hemoglobin is above 105 g/L.
   - ESR is 30 mm/hour or less.

2. Severe ulcerative colitis is identified if:
   - Bowel movements are 6 or more.
   - Blood in stool is visible macroscopic blood.
   - Temperature is 37.8 degrees C or higher.
   - Pulse exceeds 90 bpm.
   - Hemoglobin is 105 g/L or less.
   - ESR exceeds 30 mm/hour.

3. In any other combination not fitting the above criteria, the condition is considered moderate ulcerative colitis.

Upon computation, the result stating the severity level (mild, moderate, severe) is returned. This assessment assists in clinical decision-making by categorizing the disease's current impact on the patient, facilitating appropriate management and treatment strategies. If any inputs are missing or not selected, an instructions prompt to complete all inputs is displayed.

The Truelove and Witts Index is a clinical calculator used to assess the severity of ulcerative colitis, a chronic inflammatory bowel disease. This index is applicable to patients diagnosed with ulcerative colitis and is used to guide treatment decisions and predict disease prognosis. 

The Truelove and Witts Index takes into account several parameters including number of bowel movements, presence of blood in stool, temperature, pulse rate, hemoglobin level, and erythrocyte sedimentation rate (ESR). Based on these parameters, the severity of ulcerative colitis is classified as mild, moderate, or severe.

The clinical utility of this calculator lies in its ability to stratify patients according to disease severity, which can help clinicians decide on the appropriate level of intervention, ranging from outpatient management to hospital admission and aggressive treatment. 

There are no specific exclusion criteria for the use of the Truelove and Witts Index. However, it may not be as accurate in patients with atypical presentations of ulcerative colitis or in those with co-existing conditions that could affect the parameters used in the index.