Does HUS Present with Prolonged PTT?
No, hemolytic uremic syndrome (HUS) does not typically present with a prolonged partial thromboplastin time (PTT). The coagulation cascade remains largely intact in HUS, distinguishing it from disseminated intravascular coagulation (DIC).
Laboratory Profile of HUS
HUS is characterized by a specific triad without significant coagulation factor consumption:
- Platelet consumption (platelets <150,000/mm³ or 25% reduction from baseline) 1
- Microangiopathic hemolytic anemia with negative direct Coombs test, elevated LDH, reduced haptoglobin, and schistocytes on peripheral smear 1
- Renal involvement with elevated creatinine, hematuria, and/or proteinuria 1
The PTT and PT remain normal in HUS, which is a critical distinguishing feature from DIC 2, 3. This reflects the fact that HUS involves platelet thrombi formation in the microvasculature without extensive activation of the soluble coagulation system 3.
Key Diagnostic Distinction: HUS vs. DIC
The most important laboratory difference between HUS and DIC is the coagulation profile:
- In HUS: PT is normal (within 5 seconds of the upper limit of normal) and PTT is normal 2
- In DIC: Both PT and PTT are prolonged due to consumption of clotting factors 2
A study specifically examining this distinction found that a PT within 5 seconds of the upper limit of normal combined with profound thrombocytopenia (<20 × 10³/μL) had 92% specificity for TTP-HUS over DIC 2. This combination is clinically useful because prompt differentiation is essential—TTP-HUS is 90% fatal without plasma exchange 2.
Rare Exception: Concurrent Factor Deficiency
In extremely rare cases, a patient with atypical HUS may coincidentally have an unrelated coagulation factor deficiency that prolongs the PTT:
- One case report documented atypical HUS with prolonged PTT due to concurrent factor XII deficiency (two novel mutations: W222G and R447S) 4
- This represents a coincidental finding rather than a feature of HUS itself 4
- Factor XII deficiency does not cause bleeding and required no treatment 4
Clinical Pitfall to Avoid
Do not delay plasma exchange in suspected HUS/TTP while waiting for ADAMTS13 results if the PT and PTT are normal. Normal coagulation times with thrombocytopenia and microangiopathic hemolytic anemia strongly favor HUS/TTP over DIC 2. The presence of normal fibrinogen levels further supports this diagnosis, as fibrinogen is almost invariably normal in HUS but consumed in DIC 3.