STEPTRACKERHEMATOLOGY | PATHOLOGY | HIGH-YIELD REVIEW

Platelet Disorders

Too few platelets (destroyed, consumed, not made, trapped) or platelets that don't work — PT and PTT stay normal.

FOCUS ON
Separate destruction (ITP, HIT) from consumption in microthrombi (TTP, HUS), then name the qualitative defects by their glycoprotein: GPIb (Bernard-Soulier) vs GPIIb/IIIa (Glanzmann). TTP and HIT are emergencies, and platelet transfusion makes them worse.
DISORDER
MECHANISM
LABS
CLUE · TREATMENT
Immune thrombocytopenia (ITP)
destruction
IgG against GPIIb/IIIa → splenic macrophages remove platelets.
Isolated ↓ platelets; ↑ megakaryocytes in marrow; PT/PTT normal
Child after viral illness (self-limited) or adult woman (chronic, SLE) · steroids, IVIG, splenectomy
TTP
consumption
ADAMTS13 deficiency (usually autoantibody) → huge vWF multimers → platelet microthrombi.
↓ platelets, schistocytes, ↑ LDH, ↑ indirect bilirubin; PT/PTT normal
Fever, neurologic signs, renal injury · emergency plasmapheresis; caplacizumab; no platelet transfusion
HUS
consumption
Shiga toxin (E coli O157:H7) damages renal endothelium → microthrombi.
↓ platelets, schistocytes, AKI; PT/PTT normal
Child after bloody diarrhea; atypical HUS = complement dysregulation
Heparin-induced thrombocytopenia
destruction + clotting
IgG against heparin–PF4 complex activates platelets.
> 50% fall in platelets 5–10 days after heparin; anti-PF4 ELISA, serotonin release assay
New thrombosis · stop all heparin → argatroban or other non-heparin anticoagulant
Bernard-Soulier syndrome
adhesion defect
GPIb deficiency → can't bind vWF · autosomal recessive.
Mild ↓ count, giant platelets; ↑ bleeding time; no agglutination with ristocetin (not fixed by normal plasma)
Mucosal bleeding from childhood
Glanzmann thrombasthenia
aggregation defect
GPIIb/IIIa deficiency → no fibrinogen bridges · autosomal recessive.
Normal count; ↑ bleeding time; no clumping on smear; ristocetin normal
Mucosal bleeding from childhood
Drug-induced dysfunction
function
Aspirin irreversibly blocks COX-1; NSAIDs reversibly; clopidogrel blocks ADP receptor.
Normal count, ↑ bleeding time
Hold before surgery
Uremic platelet dysfunction
function
Uremic toxins impair adhesion and aggregation.
Normal count, ↑ bleeding time
Desmopressin, dialysis
Production & sequestration
other causes
Marrow failure, infiltration, alcohol, B12/folate deficiency · splenomegaly traps platelets.
↓ megakaryocytes (production) · normal marrow + big spleen (sequestration)
Cirrhosis with portal hypertension; aplastic anemia

Mechanism — where platelets are lost or fail

AdhesionGPIb ↔ vWF · Bernard-Soulier, vWD
→
ActivationTXA₂, ADP · aspirin, uremia
→
AggregationGPIIb/IIIa · Glanzmann, ITP target
DestroyedITP, HIT
ConsumedTTP, HUS, DIC
Not madeMarrow failure
TrappedHypersplenism

Microangiopathies compared

FEATURE
TTP
HUS
DIC
Platelets
↓
↓
↓
Schistocytes
Yes
Yes
Yes
PT / PTT
Normal
Normal
↑
Key cause
ADAMTS13 ↓
Shiga toxin
Systemic factor consumption
Main organ
Brain + kidney
Kidney
Widespread
Treatment
Plasmapheresis
Supportive, dialysis
Treat the cause

ITP has low platelets without schistocytes.

How it's tested

COMMON CONFUSIONTTP and HUS look similar. TTP leans toward neurologic signs and low ADAMTS13. HUS leans toward a child, bloody diarrhea, then kidney failure.
THE TRAPA platelet count that halves on day 7 of heparin with a new DVT is HIT. Giving platelets or switching to low-molecular-weight heparin is wrong.
YOUR APPROACHCount low? Check the smear (schistocytes or not), then the PT/PTT. Count normal but bleeding? Check bleeding time, then ristocetin and clumping.

If you see… think…

Child with petechiae 2 weeks after a cold, only platelets low → acute ITP
Fever, confusion, schistocytes, low platelets, normal PT/PTT → TTP — plasmapheresis
Child, bloody diarrhea after undercooked burger, then AKI → HUS
Platelets halve on day 7 of heparin + new clot → HIT — argatroban
Giant platelets, failed ristocetin agglutination → Bernard-Soulier (GPIb)
Normal count, no clumping on smear → Glanzmann (GPIIb/IIIa)
Dialysis patient oozing, long bleeding time → uremic dysfunction — desmopressin
Cirrhosis with big spleen and mild thrombocytopenia → sequestration
Platelet transfusion would worsen → TTP and HIT