Laboratory Indicators of Active Bleeding
Repeated hemoglobin (Hb) and hematocrit (Hct) measurements are the primary laboratory markers for detecting bleeding, as a single initial value in the normal range can mask early-phase hemorrhage. 1
Core Laboratory Tests for Bleeding Detection
Hemoglobin and Hematocrit Monitoring
Serial Hb/Hct measurements are essential because initial values may appear falsely normal before plasma equilibration occurs, typically masking acute blood loss in the first 30-60 minutes after bleeding begins 1, 2
Hemoglobin drops can occur within minutes of injury in actively bleeding patients, with Hb ≤10 g/dL measured within 30 minutes of arrival correctly identifying significant bleeding in 87% of trauma patients 2
A decline in Hb of ≥2 g/dL between measurements strongly suggests ongoing hemorrhage, particularly when accounting for fluid resuscitation volumes 3, 4
Low initial Hct or Hb levels closely correlate with hemorrhagic shock, with Hb <80 g/L (8 g/dL) in pelvic trauma patients associated with non-survival 1
Complete Blood Count (CBC)
Obtain CBC with platelet count to assess for anemia (low Hb/Hct) and thrombocytopenia, both of which indicate either blood loss or impaired hemostasis 1, 5
Leukocytosis ≥11,000/mm³ is an independent predictor of significant hemoglobin drops (≥2 g/dL), suggesting inflammatory response to bleeding or tissue injury 3
Coagulation Studies
Prothrombin time (PT) or INR and activated partial thromboplastin time (aPTT) should be obtained in bleeding patients to detect coagulopathy that may worsen hemorrhage 1, 5
Platelet count, PT/INR, and aPTT form the basic coagulation panel, with additional tests including fibrinogen level, thrombin time, and D-dimers when coagulopathy is suspected 1, 6
Fibrinogen levels <1.5 g/L indicate consumptive coagulopathy and predict ongoing bleeding requiring replacement therapy 7
Additional Laboratory Markers
Metabolic Indicators of Hemorrhagic Shock
Lactate >2 mmol/L measured via venous blood gas indicates tissue hypoperfusion from hemorrhagic shock 7
Base deficit and pH correlate with severity of blood loss, with worsening base deficit and decreasing pH associated with lower Hb levels and increased transfusion requirements 2
Blood urea nitrogen (BUN) and creatinine should be checked to assess renal perfusion and guide fluid resuscitation 1
Specialized Testing for Bleeding Disorders
Von Willebrand factor (VWF) testing includes VWF antigen (VWF:Ag), VWF ristocetin cofactor activity (VWF:RCo), and factor VIII coagulant activity (FVIII) when mucocutaneous bleeding history is strong 1, 5
A VWF:RCo to VWF:Ag ratio <0.5-0.7 suggests von Willebrand disease, though this is rarely the cause of intracranial hemorrhage or severe bleeding 1
Critical Interpretation Considerations
Timing and Serial Measurements
The key pitfall is relying on a single initial Hb/Hct value, which frequently appears normal despite significant ongoing bleeding because plasma equilibration requires time and is confounded by crystalloid resuscitation 1, 2
Point-of-care Hb measured prehospital has modest predictive ability (AUC 0.72) with best cutoffs of 12 g/dL for women and 13 g/dL for men, while hospital admission Hb has superior predictive ability (AUC 0.92) with cutoffs of 10 g/dL for women and 12 g/dL for men 4
The magnitude of Hb drop between prehospital and hospital measurements (DeltaPOC-Hb ≥2 g/dL) predicts significant hemorrhage better than shock index alone, even accounting for fluid resuscitation volumes 4
False Results and Confounders
Prolonged PT and aPTT from parenchymal liver damage in abusive head trauma should not automatically be interpreted as primary bleeding disorders 1
Hemodilution from crystalloid resuscitation artificially lowers Hb/Hct and can mask the true degree of blood loss or exaggerate it depending on timing 1, 2, 8
VWF is an acute phase reactant that can be falsely elevated by stress, inflammation, pregnancy, or estrogen use, requiring up to 3 repeat measurements for reliable results 1, 5