Laboratory Testing for Diarrhea with Hematochezia
Yes, obtaining a CBC with platelet count and fecal analysis is absolutely appropriate and recommended for a patient presenting with diarrhea and hematochezia, even after just 2 episodes. These tests are essential for risk stratification, detecting complications, and guiding management decisions.
Complete Blood Count (CBC) with Platelet Count
A CBC is clinically useful and should be obtained to assess for anemia from blood loss, evaluate hydration status, and detect early signs of serious complications 1.
- Hemoglobin and hematocrit assessment helps determine the severity of bleeding and need for transfusion 1
- White blood cell count and differential can suggest bacterial etiology when elevated, though it should not be used alone to establish the specific cause of diarrhea 1
- Platelet count monitoring is critical because a declining platelet trend during days 1-14 of diarrheal illness indicates increased risk of developing hemolytic uremic syndrome (HUS), particularly if Shiga toxin-producing E. coli (STEC) infection is present 1
Critical Monitoring for HUS
If STEC infection is diagnosed or suspected (especially with bloody diarrhea), frequent monitoring of hemoglobin, platelet counts, electrolytes, BUN, and creatinine is strongly recommended to detect early manifestations of HUS before renal injury occurs 1. Daily monitoring should continue until the platelet count begins to increase or stabilize with resolved symptoms 1.
Fecal Analysis
Fecal analysis is appropriate and should include stool culture for bacterial pathogens and testing for specific organisms based on clinical context 1.
- Stool culture for Salmonella, Shigella, Campylobacter, and STEC is indicated for patients with bloody diarrhea 1, 2
- Shiga toxin testing should be performed given the hematochezia, as STEC infections can lead to HUS 1
- C. difficile testing should be included if there has been any antibiotic exposure in the preceding 8-12 weeks 2, 3
What NOT to Order
Fecal leukocyte examination and stool lactoferrin detection should NOT be used to establish the cause of acute infectious diarrhea 1. There is insufficient data to recommend fecal calprotectin measurement in acute infectious diarrhea 1.
Additional Essential Laboratory Tests
Beyond CBC and stool studies, obtain electrolytes, BUN, and creatinine to assess for dehydration and renal function 1, 2.
- Coagulation assessment should be checked to evaluate bleeding risk, especially if bleeding is severe 1
- Blood type and cross-match should be ordered if there are signs of severe bleeding requiring potential transfusion 1
Risk Stratification Based on Findings
Vital signs assessment is mandatory to determine hemodynamic stability and severity of bleeding 1. The combination of vital signs, hemoglobin/hematocrit, presence of blood on rectal examination, and comorbidities helps stratify risk 1.
Red Flags Requiring Hospitalization
- Fever >38.5°C with bloody stools suggests invasive bacterial infection 2
- Signs of dehydration (dry mucous membranes, decreased skin turgor, orthostatic changes) 2
- Severe abdominal pain or distention 2
- Inability to tolerate oral fluids 2
- Immunocompromised state 2
Common Pitfalls to Avoid
Do not wait for test results to initiate supportive care - begin fluid resuscitation immediately based on clinical assessment 2. A near-normal hemoglobin may actually suggest dehydration rather than absence of bleeding 1.
Do not use peripheral white blood cell count alone to determine etiology - while it may suggest bacterial infection when elevated, it cannot establish a specific diagnosis and should not delay appropriate stool testing 1.
Do not assume a single CBC is sufficient if STEC is suspected - serial monitoring is essential as HUS evolves over time, and a declining platelet trend is the key warning sign 1.