Does Syncope Lower Hemoglobin Levels?
No, syncope itself does not lower hemoglobin levels—syncope is a symptom of transient loss of consciousness due to reduced cerebral blood flow, not a cause of anemia. However, acute blood loss causing anemia can trigger syncope through hemorrhagic shock, making hemoglobin measurement useful only when clinical assessment suggests bleeding as the underlying cause.
When Hemoglobin Testing is Appropriate in Syncope
Hemoglobin/hematocrit determination may confirm suspected acute blood loss, but examination of stool for blood may be more accurate as hemoglobin and hematocrit may be normal early in acute blood loss 1
Targeted blood tests are reasonable in the evaluation of selected patients with syncope identified on the basis of clinical assessment from history, physical examination, and ECG 1
Routine and comprehensive laboratory testing, including hemoglobin, is not recommended (Class III: No Benefit) in the evaluation of all patients with syncope 1
Clinical Scenarios Where Hemoglobin is Relevant
Look for specific clinical indicators of blood loss:
- History of peptic ulcer disease or tarry stools 1
- Orthostatic hypotension on physical examination suggesting volume depletion 1
- Abdominal pain with syncope (rare cases of intra-abdominal hemorrhage) 2
- Severe anemia presenting with syncope and GI bleeding 3
The Diagnostic Yield Problem
Blood tests rarely yield diagnostically useful information when used routinely in syncope evaluation, with diagnostic yield as low as 5% 1
The broad-panel testing common in clinical practice at the point of triage has no data supporting its utility 1
Nonfocused additional testing in patients with syncope does not improve diagnostic yield, however it does increase hospitalization rate and significantly increases cost 1
Common Pitfall to Avoid
Do not order hemoglobin reflexively in all syncope patients. The key error is ordering comprehensive laboratory panels without clinical suspicion of a specific diagnosis. Early in acute hemorrhage, hemoglobin may appear falsely normal because hemodilution has not yet occurred 1. If bleeding is suspected based on history and examination, stool guaiac testing may be more immediately informative than hemoglobin 1.
The Correct Diagnostic Approach
Base testing decisions on the initial clinical assessment:
- History and physical examination identify the cause in up to 85% of cases where a diagnosis is established 1
- A standard 12-lead ECG is warranted when history and physical examination do not reveal an etiology or when cardiovascular disease is suspected 1
- Specific blood testing should stem from assessment by history and physical examination when the nature of the syncope presentation or associated comorbidities suggests a diagnostic role 1