AIRD REUSABLE BLOCKS
A 24 year-old woman with a known history of sickle cell disease presented to an outside hospital (OSH) with a two-day history of progressive shortness of breath, chest pain, and fever. She also complained of productive cough with green sputum. Chest X ray showed a left lower lobe infiltrate. She was started on antibiotics (ceftriaxone and azithromycin). Her Hb and platelet count started to fall, so she was transferred to your institution for higher level of care. She has had multiple past admissions for pain crises and acute chest syndrome. She also has a history of osteonecrosis and had a cholecystectomy in the past. Her family history is positive for sickle cell disease. She is a non-smoker and drinks socially. She is currently on disability. Medications at time of initial presentation to OSH included folic acid and hydroxyurea. She has no known allergies.
The following describes this patient’s physical exam when you see her in the emergency room:
General appearance: In visible distress from pain
Vital signs: Heart rate 120/min, blood pressure 110/65, respiratory rate 18/min, T 100oF, oxygen saturation 84% on room air
Head and neck: No lymphadenopathy
CVS: Normal heart sounds, no murmurs
Chest: Normal to inspection, palpation, percussion, and auscultation
Abdomen: Soft, non-tender, no hepatosplenomegaly
CNS: No focal changes
Let’s look at how a PT and aPTT are carried out:
Notes:
- Step 1 – Draw whole blood from a blood vessel (typically an arm vein) into a blue top tube containing the anticoagulant, sodium citrate.
- Step 2 – Spin the liquid blood sample in a centrifuge so that red cells layer on the bottom, plasma on the top.
- Step 3 – Pipette plasma (top layer) into a clean, empty test tube.
- Step 4 – Add phospholipid, calcium and activator (tissue factor for PT, silica for aPTT).
- Step 5 – Incubate at 37 degrees C and measure time to clot formation, either with machine or more rarely by vision.
Note that the major difference between the PT and aPTT is the nature of the activator added to the sample.
Further work-up of this case requires some knowledge of the clotting cascade. Let’s review the basics:
Notes:
- The bottom line is formation of an insoluble fibrin plug, which, along with platelets, stems blood loss.
- Fibrin is derived from thrombin-mediated cleavage of soluble fibrinogen (which is a structural protein).
- Thrombin (which is a type of enzyme called a serine protease) is formed when another serine protease, activated factor X (Xa), cleaves prothrombin.
- Two pathways may activate factor X:
- The intrinsic pathway, which consists of a series of linked reactions involving serine proteases FXII, FXI and FIX, and cofactor FVIII.
- The extrinsic pathway, which consists of tissue factor-mediated activation of FVII (FVIIa).
- In vivo, the clotting cascade is always initiated by tissue factor activation of FVII (extrinsic pathway) and amplified by the intrinsic pathway via cross-talk (FVIIa activates FIX) and feedback (thrombin activates FXI and FVIII) mechanisms.
- aPTT measures the integrity of the intrinsic pathway.
- PT/INR measures the integrity of the extrinsic pathway.
According to the scheme above, an isolated elevation in the aPTT indicates a deficiency of or inhibitor against a clotting factor in the intrinsic pathway, namely FXII, FXI, FIX or FVIII.
