ITP vs TTP vs HUS vs DIC: How to Differentiate Them on Board Exams

The fastest way to differentiate ITP, TTP, HUS, and DIC on board exams is with three labs and one trigger. Look at the smear: if there are no schistocytes and the platelets are isolated low, that's ITP. If there are schistocytes and PT/PTT/fibrinogen are normal, that's TTP or HUS. If there are schistocytes and PT/PTT are prolonged with low fibrinogen, that's DIC. Then split TTP from HUS by which organ is failing: neuro symptoms point to TTP, severe acute kidney injury (especially in a child after bloody diarrhea) points to HUS.

These four entities all involve thrombocytopenia, so the COMLEX and USMLE write them as differentials in the same vignette over and over. The question writers know that students who guess on the platelet number alone get burned. The trick on every one of these questions is the pattern of the smear and the coagulation panel together, plus the demographic and trigger in the stem. Lock that pattern in once and these questions stop being a coin flip.

Why these four diagnoses get confused on boards

Every one of them presents with low platelets. That's the trap. NBOME and USMLE writers love a stem that opens with "platelet count of 18,000" because they know students will collapse the differential into a single answer. The discriminating features come from the rest of the panel and the clinical context, not the platelet number.

The four conditions also overlap on bleeding signs. Petechiae, purpura, and mucocutaneous bleeding can show up in any of them. So the question is never "does this patient have low platelets," it's "why are the platelets low and what is the rest of the picture telling you."

Three frameworks make this easy:

1. Smear: schistocytes present or absent
2. Coag panel: PT, PTT, fibrinogen, D-dimer normal or abnormal
3. Trigger and demographic: child after diarrhea, postpartum hemorrhage, sepsis in the ICU, otherwise well young woman with neuro symptoms

Once you can map a stem onto those three axes, the answer falls out fast.

ITP: isolated thrombocytopenia, no schistocytes

What ITP actually is

Immune thrombocytopenia (formerly idiopathic thrombocytopenic purpura, still abbreviated ITP) is autoimmune destruction of platelets. The mechanism is IgG autoantibodies against platelet glycoproteins, most commonly GPIIb/IIIa. Splenic macrophages bind the antibody-coated platelets and destroy them. Bone marrow tries to compensate by ramping up production, which is why young, large platelets often show up on the smear.

ITP is not a microangiopathy. There's no schistocyte formation, no consumption coagulopathy, and no organ ischemia from microthrombi. It's just a low platelet count in an otherwise well patient.

Board demographics and triggers for ITP

There are two classic stems:

• Pediatric ITP: a previously healthy child develops petechiae and bruising 1 to 3 weeks after a viral illness. Often self-limited. Many cases resolve without treatment.
• Adult ITP: a young to middle-aged woman with insidious onset of bruising, petechiae, gum bleeding, or heavy menses. Often chronic. Sometimes triggered by HIV, hepatitis C, H. pylori, lupus, or medications like quinine, vancomycin, or heparin (HIT is its own beast).

Board labs for ITP

The lab pattern is intentionally boring. The diagnosis is clinical and one of exclusion. There's no confirmatory test for routine cases.

Board treatment for ITP

ITP treatment is high-yield on COMLEX Level 1 and USMLE Step 1, and even more so on COMLEX Level 2-CE and USMLE Step 2 CK where the management questions get more clinical.

The buzzwords COMLEX and USMLE love: "petechiae and purpura in a previously healthy child after a viral URI" and "isolated thrombocytopenia with otherwise normal CBC and coag panel."

TTP: schistocytes, neuro symptoms, normal coags

What TTP actually is

Thrombotic thrombocytopenic purpura is a thrombotic microangiopathy caused by deficiency of ADAMTS13, the metalloprotease that cleaves von Willebrand factor multimers. When ADAMTS13 is low (acquired autoantibody in most adults; congenital in Upshaw-Schulman syndrome), ultra-large vWF multimers persist in circulation. Platelets aggregate on these multimers in small vessels. The aggregates shred passing red cells, producing schistocytes, and chew through the platelet pool.

This is a true emergency. Untreated TTP has a mortality over 90 percent. Treated, it drops to about 10 to 20 percent. The treatment must start before the lab confirmation comes back.

The classic TTP pentad and why it's misleading

Textbooks teach a pentad: microangiopathic hemolytic anemia (MAHA) with schistocytes, thrombocytopenia, neurologic symptoms, renal dysfunction, and fever. The full pentad shows up in fewer than 10 percent of real patients. Modern guidelines treat MAHA plus thrombocytopenia (without another explanation) as enough to suspect TTP and start plasma exchange.

On boards, you'll usually see three or four of the five. Neuro symptoms (headache, confusion, focal deficits, seizure) are the high-yield differentiator from HUS. Renal involvement in TTP is usually mild to moderate. Fever may or may not be there.

Board demographics and triggers for TTP

The stem is almost always an adult, more commonly a woman. Triggers include pregnancy or postpartum, autoimmune disease (especially lupus), HIV, certain medications (clopidogrel, ticlopidine, quinine, calcineurin inhibitors), and sometimes idiopathic.

Board labs for TTP

Board treatment for TTP

TTP management is heavily tested on Level 1 / Step 1 and even more so on COMLEX Level 2-CE and USMLE Step 2 CK, where the "next step" question targets the urgency of plasma exchange.

If a stem describes MAHA plus thrombocytopenia plus neuro symptoms in an adult woman and asks for the next step, the answer is plasma exchange. Anything else is a distractor.

HUS: schistocytes, kidney failure, often a child with bloody diarrhea

What HUS actually is

Hemolytic uremic syndrome is also a thrombotic microangiopathy, but the trigger is different from TTP. The classic typical HUS is caused by Shiga toxin produced by Shiga toxin-producing E. coli (STEC), most famously serotype O157:H7. The toxin damages renal endothelium, triggering platelet activation and aggregation, MAHA, and acute kidney injury concentrated in the kidneys.

There's also atypical HUS, driven by complement system dysregulation rather than Shiga toxin. It tends to be relapsing, can occur at any age, and is treated with eculizumab.

Board demographics and triggers for HUS

The classic STEC HUS stem: a young child with bloody diarrhea about a week after eating undercooked ground beef, drinking unpasteurized milk, or attending a petting zoo. The diarrhea precedes the renal failure by 5 to 10 days.

Atypical HUS shows up in stems where there's no diarrhea, the patient has a family history of similar episodes, or the question signals complement involvement.

Board labs for HUS

The lab pattern looks like TTP except renal failure dominates and neuro symptoms are usually absent or mild.

Board treatment for HUS

HUS management questions show up on COMLEX Level 1 / USMLE Step 1 and again on COMLEX Level 2-CE / USMLE Step 2 CK, where the same antibiotic-avoidance trap gets tested in a more clinical wrapper.

The high-yield trap: students see "low platelets, schistocytes, kidney problem" and pick plasma exchange, thinking TTP. In a child with bloody diarrhea, that's HUS, and plasma exchange is not the answer.

DIC: schistocytes plus consumption coagulopathy

What DIC actually is

Disseminated intravascular coagulation is widespread, pathologic activation of the coagulation cascade. Tissue factor or other procoagulant signals trigger systemic clotting. The body burns through clotting factors and platelets faster than it can replace them. The result is the paradox of simultaneous thrombosis and bleeding: microthrombi in small vessels, plus uncontrolled bleeding from sites that should hold a clot.

DIC is always secondary to something else. It's never a primary diagnosis. The board task is usually to recognize the lab pattern and identify or treat the underlying cause.

Board demographics and triggers for DIC

Common triggers on board stems:

• Sepsis, especially gram-negative bacteremia
• Major trauma or burns
• Obstetric emergencies: amniotic fluid embolism, abruptio placentae, severe preeclampsia/HELLP, retained fetus
• Malignancy, especially acute promyelocytic leukemia (APL, M3) with t(15;17)
• Snake envenomation, hemolytic transfusion reactions, severe pancreatitis

Board labs for DIC

This is the lab pattern that separates DIC from everything else.

If you see thrombocytopenia plus prolonged PT/PTT plus low fibrinogen plus high D-dimer, that's DIC. Period. The smear may show schistocytes, which puts it on the same axis as TTP and HUS, but the coag panel is the giveaway.

Board treatment for DIC

DIC management is tested across COMLEX Level 1, USMLE Step 1, COMLEX Level 2-CE, and USMLE Step 2 CK. The Level 2 / Step 2 questions usually test product-replacement decisions and underlying-cause management, so this is worth memorizing twice.

Side-by-side comparison table

This is the table to drill until it's automatic. If you can fill it out from memory, you can answer most board questions on this differential in under 30 seconds.

A note on the Coombs test: all four conditions are Coombs-negative. If the smear shows hemolysis and Coombs is positive, you're in autoimmune hemolytic anemia territory, not in this differential.

Five stem clues that crack these questions fast

These are the patterns that show up over and over on COMLEX and USMLE questions. Internalize them and the questions get fast.

1. No schistocytes on smear plus normal coags equals ITP. If the smear is described as low platelets only, the differential collapses to ITP. The trigger is usually viral illness in a child or autoimmune flare in an adult.

2. Schistocytes plus normal PT/PTT plus neuro symptoms in an adult woman equals TTP. The next-step answer is plasma exchange. Always.

3. Schistocytes plus normal PT/PTT plus severe AKI plus a recent diarrheal illness in a child equals HUS. The next step is supportive care. Avoid antibiotics in the pediatric STEC scenario.

4. Schistocytes plus prolonged PT/PTT plus low fibrinogen plus high D-dimer equals DIC. Treat the underlying cause and replace products if bleeding.

5. Pregnancy and platelets: pregnancy adds a few extra players you have to rule in or out. Gestational thrombocytopenia is the most common cause of mild thrombocytopenia in pregnancy, usually third trimester, not associated with MAHA, and resolves after delivery. HELLP syndrome (hemolysis, elevated liver enzymes, low platelets) overlaps with TTP and DIC and is treated by delivery. ITP can occur in pregnancy and is managed similarly to non-pregnant patients. AFLP (acute fatty liver of pregnancy) overlaps with HELLP and DIC. The differentiator is usually the timing (third trimester or postpartum), the LFTs, and the rest of the systemic picture.

Common board pitfalls and how to avoid them

Pitfall 1: Platelet transfusion in TTP or HUS. Students see "platelets 12,000 with bleeding" and reach for platelets. In TTP and HUS, transfused platelets feed microthrombi. The answer is plasma exchange (TTP) or supportive care (HUS), and platelet transfusion is reserved for life-threatening bleeding only.

Pitfall 2: Antibiotics in pediatric STEC HUS. The reflex to treat infection with antibiotics is wrong here. Antibiotics may increase Shiga toxin release. Supportive care is the answer.

Pitfall 3: Calling DIC just on the basis of low platelets. DIC requires the consumption pattern: low platelets plus prolonged PT/PTT plus low fibrinogen plus high D-dimer. Without the coag derangement, it isn't DIC.

Pitfall 4: Forgetting that ADAMTS13 confirms but doesn't rule in TTP fast enough. ADAMTS13 takes time. The clinical diagnosis of MAHA plus thrombocytopenia in the right patient is enough to start plasma exchange. Boards reward the early-treatment answer.

Pitfall 5: Treating ITP in a stable patient. Mild ITP, especially in children after a viral illness, often does not need treatment. The right answer can be observation. Don't reach for steroids reflexively.

Pitfall 6: Missing APL as the DIC cause. A young adult with bleeding from multiple sites, low fibrinogen, prolonged PT/PTT, and "blasts on peripheral smear" or "Auer rods" is acute promyelocytic leukemia (M3) with t(15;17). The treatment is all-trans retinoic acid (ATRA) emergently, plus supportive blood products. APL is one of the few oncologic emergencies where DIC is the presenting feature.

Practice questions

These two questions test the differential pattern directly. Cover the answer choices, work through the stem, then check yourself.

Question 1

A 7-year-old boy is brought to the emergency department for decreased urine output and lethargy. His mother reports that he developed bloody diarrhea about 8 days ago, which has since resolved. Vital signs include temperature 37.4°C (99.3°F), heart rate 124/min, and blood pressure 102/65 mm Hg. Physical examination shows pallor, mild scleral icterus, and scattered petechiae on the lower extremities. Laboratory studies show a hemoglobin of 7.8 g/dL, platelet count of 38,000/mm³, creatinine of 3.4 mg/dL, lactate dehydrogenase of 1,420 U/L, and haptoglobin below the lower limit of detection. The peripheral smear shows fragmented red cells. Prothrombin time and partial thromboplastin time are within reference range. Which of the following is the most appropriate next step in management?

A. Administer intravenous ceftriaxone empirically B. Administer intravenous immunoglobulin C. Begin empiric oral corticosteroid D. Begin supportive intravenous hydration E. Initiate urgent plasma exchange

Correct answer: D

This child has hemolytic uremic syndrome following Shiga toxin-producing E. coli infection. The diagnostic pattern is the triad of microangiopathic hemolytic anemia (low hemoglobin, fragmented red cells, elevated LDH, low haptoglobin), thrombocytopenia, and acute kidney injury, with normal coagulation studies. Management of pediatric STEC HUS is supportive: hydration, electrolyte correction, and dialysis if needed. Antibiotics are avoided in this setting because they may increase Shiga toxin release and worsen outcomes (A is wrong). IVIG and corticosteroids are not first-line in HUS (B and C are wrong). Plasma exchange is the answer for TTP, which is distinguished by neurologic involvement and an adult demographic; the lack of neuro findings, the diarrheal prodrome, and the pediatric age make HUS the diagnosis here, not TTP (E is wrong).

Question 2

A 32-year-old woman is admitted to the intensive care unit 6 hours after vaginal delivery complicated by placental abruption. She is bleeding from her intravenous catheter sites, surgical incision, and gums. Vital signs include temperature 37.0°C (98.6°F), heart rate 132/min, blood pressure 88/52 mm Hg, and respiratory rate 24/min. Laboratory studies show hemoglobin of 7.2 g/dL, platelet count of 42,000/mm³, prothrombin time of 28 seconds (reference range 11 to 13 seconds), partial thromboplastin time of 62 seconds (reference range 25 to 35 seconds), fibrinogen of 78 mg/dL (reference range 200 to 400 mg/dL), and D-dimer markedly elevated. Peripheral smear shows fragmented red cells. Which of the following best explains the underlying mechanism of these laboratory findings?

A. Antibody-mediated platelet destruction B. Complement-mediated endothelial injury C. Deficiency of ADAMTS13 metalloprotease D. Shiga toxin-induced endothelial damage E. Widespread tissue factor activation

Correct answer: E

The lab pattern is the consumption coagulopathy of disseminated intravascular coagulation: thrombocytopenia, prolonged PT and PTT, low fibrinogen, markedly elevated D-dimer, and microangiopathic hemolysis with fragmented red cells. The clinical setting (postpartum hemorrhage with placental abruption) is a classic obstetric DIC trigger. The underlying mechanism is widespread exposure of tissue factor from injured placental tissue and amniotic fluid, which activates the extrinsic coagulation cascade systemically and consumes platelets, clotting factors, and fibrinogen faster than they can be replaced (E is correct). Antibody-mediated platelet destruction is the mechanism of ITP, which produces isolated thrombocytopenia with normal coagulation and no schistocytes (A is wrong). Complement-mediated endothelial injury underlies atypical HUS and produces a thrombotic microangiopathy with normal coagulation studies (B is wrong). ADAMTS13 deficiency produces TTP, with normal PT, PTT, and fibrinogen (C is wrong). Shiga toxin-induced endothelial damage produces typical HUS, with renal-dominant injury after a diarrheal prodrome and normal coagulation studies (D is wrong).

Frequently asked questions about ITP, TTP, HUS, and DIC

What is the single fastest way to differentiate TTP from HUS on board exams?

The fastest discriminator is which organ is failing. TTP almost always has neurologic involvement (headache, confusion, focal deficits, or seizure) and renal involvement is mild to moderate. HUS is dominated by acute kidney injury and neuro findings are usually absent. Demographics help too: TTP is most often an adult woman; HUS is most often a child after bloody diarrhea, especially within a week of eating undercooked beef or attending a petting zoo. ADAMTS13 activity below 10 percent confirms TTP if the result is available, but treatment doesn't wait on the assay.

Why are platelet transfusions avoided in TTP and HUS?

Both conditions involve microthrombi in small vessels. Transfused platelets can be incorporated into existing thrombi, fueling more occlusion and worsening organ ischemia. Platelet transfusion is reserved for life-threatening bleeding. This is one of the highest-yield "next step" traps in this differential, and boards test it frequently. The answer to a TTP stem with low platelets is plasma exchange, not platelet transfusion.

How can I quickly distinguish DIC from TTP and HUS?

The coagulation panel. DIC consumes clotting factors, so PT and PTT are prolonged and fibrinogen is low. TTP and HUS leave the coagulation panel intact: normal PT, normal PTT, normal fibrinogen. D-dimer is markedly elevated in DIC and only mildly elevated, if at all, in TTP and HUS. If the stem gives you all four labs, you can split DIC from the microangiopathies in one glance.

Why does the question always test ITP in a child after a viral illness?

It's the highest-yield clinical pattern. Pediatric ITP is most often a self-limited postinfectious autoimmune phenomenon, and boards reward the recognition of "previously well child plus recent URI plus petechiae plus isolated thrombocytopenia." It also tests the lower-stakes management answer: observation rather than treatment. Adult ITP is tested too, usually in a young woman with chronic mucocutaneous bleeding or heavy menstrual bleeding and an isolated low platelet count.

What's the link between APL and DIC, and why does it matter on boards?

Acute promyelocytic leukemia (APL, AML M3) with the t(15;17) translocation classically presents with DIC at diagnosis. The promyelocytes release procoagulant material that triggers consumption coagulopathy. The board task is to recognize the pattern (young adult with bleeding, low fibrinogen, blasts or Auer rods on smear) and identify ATRA (all-trans retinoic acid) as the urgent treatment alongside supportive blood products. APL is one of the few hematologic emergencies where treatment of the cancer also resolves the coagulopathy.

Are these the only causes of MAHA plus thrombocytopenia I need to know?

For most board questions, TTP, HUS, and DIC cover the differential. A few other entities to keep on the list for completeness: HELLP syndrome (in pregnancy), malignant hypertension, mechanical heart valve hemolysis (usually without significant thrombocytopenia), scleroderma renal crisis, and drug-induced thrombotic microangiopathy (gemcitabine, calcineurin inhibitors). The pattern is the same (schistocytes plus low platelets) and the trigger or context separates them.

How heavily are these tested on COMLEX vs USMLE?

Both exams test this differential heavily because it integrates pathophysiology, clinical reasoning, and pharmacology in one question. COMLEX favors the classic vignette pattern (child with bloody diarrhea for HUS, adult woman with neuro symptoms for TTP, postpartum hemorrhage for DIC). USMLE Step 1 tests the same patterns plus more pathophysiology depth (ADAMTS13 mechanism, complement in atypical HUS, t(15;17) and APL). If you can fill out the comparison table from memory, you'll handle questions on either exam.

Related guides and video resources

• How long should I study for COMLEX Level 1?
• How to use Anki effectively for COMLEX
• Doctor Lucas DO on YouTube: heme/onc question breakdowns and high-yield clinical comparison videos