The essential laboratory workup in the management of lower gastrointestinal bleeding includes a complete blood count, renal function test, liver function tests, and coagulation studies. In patients with life-threatening bleeding, although not diagnostic, a blood type and crossmatch should be done.

Laboratory findings in patients presenting with lower gastrointestinal bleeding include:^{[1][2][3][4][5]}

• Complete blood count may show a low hemoglobin level or a drop from a previous baseline level.
• In acute blood loss, the initial hemoglobin level may be normal but will fall with fluid resuscitation.
• Other abnormalities, such as thrombocytopenia, may point to a variceal source of bleeding.
• The presence of uremia or a history of aspirin or clopidogrel can significantly affect platelet function without causing thrombocytopenia.
• Leukocytosis may point to an infectious or inflammatory cause.

• Abnormal values of renal function tests may indicate underlying kidney disease. Chronic kidney disease CKD is associated with increased risk for gastrointestinal bleeding by disrupting platelet function.
• The presence of uremia may suggest bleeding is from an upper gastrointestinal source.
• Intravenous contrast for angiograms must be administered with caution in patients with renal impairment to avoid the risk of contrast nephropathy.

• Patients with an underlying liver disease are at increased risk of gastrointestinal bleeding.
• Bleeding will be more difficult to control due to coagulopathy associated with liver dysfunction. Hence LFT’s are recommended to assess the severity of liver damage.
• Abnormal liver function without any previous history of liver disease may suggest the presence of colorectal varices.

• An elevated INR may indicate anticoagulation with warfarin or may be evidence of severe liver dysfunction.
• A prolonged aPTT is seen in anticoagulation with heparin.

• Blood type and cross match identifies blood groups A, B, AB, O and Rhesus (Rh) factor.
• Blood type and cross match is essential in the management of hemodynamically unstable patients who may need a blood transfusion.

• Fecal occult blood test (FOBT’s) have sufficient sensitivity to detect bleeding that is not visible in the stool.
• Three types of FOBT are currently employed.
  • Guaiac-based tests
  • Heme-porphyrin tests
  • Immuno-chemical tests