What Does "HIT" Mean in Laboratory Reports?
When a laboratory report indicates hemoglobin is "HIT," this refers to Heparin-Induced Thrombocytopenia (HIT), not the hemoglobin value itself—this is a critical diagnostic entity involving platelet count drops in patients exposed to heparin, not a hemoglobin measurement.
Understanding the Terminology
The term "HIT" in medical contexts specifically denotes Heparin-Induced Thrombocytopenia, a rare but potentially life-threatening immune-mediated reaction to heparin therapy that occurs in fewer than 1 in 1500 hospital admissions 1. This is distinct from hemoglobin measurements and represents a completely different clinical entity.
Key Diagnostic Features of HIT:
- Platelet count drops (typically ≥50% from baseline), not hemoglobin changes
- Occurs in patients receiving heparin (unfractionated or low-molecular-weight heparin)
- Develops through antibody formation against heparin-platelet factor 4 complexes 2
- Creates a prothrombotic state despite low platelets, paradoxically increasing clot risk
Clinical Phases of HIT 1:
Acute HIT:
- Decreased platelet count
- Positive functional and immunoassays
- Highly prothrombotic phase until platelet recovery
Subacute HIT A:
- Normal platelet count
- Positive functional assay still present
- Continued thrombotic risk
Subacute HIT B:
- Normal platelet count
- Negative functional assay but positive immunoassay
Remote HIT:
- All antibodies undetectable
- Past history only
Diagnostic Approach 1:
Use the 4Ts scoring system (not gestalt assessment) to estimate HIT probability:
- Thrombocytopenia severity and timing
- Timing of platelet count fall
- Thrombosis or other sequelae
- oTher causes for thrombocytopenia
For intermediate or high 4Ts scores:
- Order immunoassay immediately
- If positive, proceed to functional assay
- Begin alternative anticoagulation while awaiting results
Critical Management Points 1:
Immediately discontinue all heparin (including flushes) when HIT is suspected—this is the most crucial intervention.
Switch to non-heparin anticoagulants:
- Direct oral anticoagulants (rivaroxaban, apixaban) are now broadly accepted even before platelet recovery (74.5% and 73.5% expert acceptance respectively) 2
- Traditional options include argatroban, bivalirudin, or danaparoid
Common Pitfall:
Do not confuse HIT with hemolysis-related hemoglobin changes. If the report actually indicates hemoglobin abnormalities with hemolysis, this involves completely different mechanisms including red blood cell destruction, elevated lactate dehydrogenase, decreased haptoglobin, and increased unconjugated bilirubin 3, 4. These conditions require entirely different diagnostic and therapeutic approaches.
Monitor platelet counts in at-risk patients receiving heparin: every other day in high-risk surgical/trauma patients, every 2-3 days in intermediate-risk patients, starting from day 4 until day 14 or heparin discontinuation 1.