Labs and Quiz

The following is the patient’s complete blood count (CBC):

WBC (109/L)Hb (g/dL)Hct (%)MCV (fL)MCHC (g/dL)RDW (%)PLT (109/L)

What’s what: WBC, white blood cell count; Hb, hemoglobin; MCV, mean cell volume; MCHC, mean cellular hemoglobin concentration; RDW, red cell distribution width; platelets, PLT; Normal values: WBC 5-10 x 109/L, RBC 4-6 x 1012/L, Hb 12-16 g/dL, Hct 35-47%, MCV 80-100 fL, MCHC 32-36 g/dL, RDW-SD < 45%, platelets (PLT) 150-450 x 109/L

Her PT and aPTT are:

PT (seconds)aPTT (seconds)

What’s what: PT, prothrombin time; INR, international ratio; aPTT, activated partial thromboplastin time; Normal values: PT 9.1-12 seconds, aPTT 24-33 seconds.

Question 1: Based on the information provided, what is the most important test to order STAT:

Peripheral blood smear
Pregnancy test
ADAMTS-13 level

Explanation to question 1:

The correct answer is B (peripheral blood smear).

While the initial thought might be to transfuse platelets in a patient with thrombocytopenia, it is imperative to rule out thrombotic thrombocytopenic purpura (TTP) in such patients, especially if they have anemia. TTP is a potentially fatal syndrome in which early recognition and therapy with therapeutic plasma exchange (TPE) are critical for patient’s survival. The presence of schistocytes in the peripheral blood smear of a patient with fever, anemia, thrombocytopenia, renal and neurological abnormalities is consistent with a diagnosis of TTP. All listed tests are important and should be ordered:

  • Creatinine to estimate the renal function.
  • ADAMTS-13 to confirm TTP.
  • Pregnancy test is important TTP may be secondary to pregnancy.

However, because there is a several day delay in getting back the ADAMTS13, examination of the peripheral blood smear is the most important next step.

Question 2: What blood products should be ordered for this patient:

Red blood cells (RBCs)
RBCs and platelets
Fresh frozen plasma

Explanation to question 2:

The correct answer is D (fresh frozen plasma).

This patient should NOT receive platelets. Platelet transfusions are contraindicated in TTP, unless the patient has a life-threatening bleeding. While red blood cells (RBCs) may be transfused later, the most important next step is to perform therapeutic plasma exchange (TPE) with fresh frozen plasma (FFP) as replacement fluid.

The Transfusion Medicine/Apheresis team will order the FFP units in a volume equivalent to one plasma volume (PV) in preparation for an emergent TPE procedure.

The exact FFP volume to be ordered is calculated via formulas incorporated in the apheresis device software or Terumo App. One would need to know the weight, height, sex, and hematocrit. A quick estimate of patient’s plasma volume (PV) can be calculated as (1-Hct) x TBV where TBV is the total blood volume estimated as 70 mL/kg x Body Weight (Kg). The amount of plasma needed in mL is divided by 250 mL (the volume of a regular FFP unit) to estimate how many units are needed or given as it is (in mL) by the Transfusion Medicine/Apheresis team to the blood bank to prepare the FFP volume needed for the procedure.

Other (relative) contraindications to platelet transfusions are:

  • Heparin-induced thrombocytopenia (HIT)
  • Immune thrombocytopenic purpura (ITP)
  • Uremia-related platelet dysfunction

Question 3: What are the correct indications for platelet transfusions (more than one answer may apply):

Patients with a platelet count less than <20 x 10^9/L
Patients with a platelet count less than <10 x 10^9/L
Bleeding patient regardless of the platelet count
Bleeding patient who takes Plavix regardless of the platelet count

Explanation to question 3:

The correct answers are B and D.

Platelets are transfused prophylactically in hospitalized patients with thrombocytopenia and:

  • Platelet count < 10 x 109/L1
  • Platelet count < 20 x 109/L if patient:
    • Is febrile
    • Is septic
    • Has mucositis
    • Undergoes a minor procedure (central venous catheter insertion or removal, biopsy)
  • Platelet count < 50 x 109/L if patient is undergoing:
    • A lumbar puncture
    • Non-minor procedure
    • Non-neuroaxial surgery
  • Platelet count < 100 x 109/L if patient undergoing:
    • Neurosurgery
    • Ophthalmologic procedures
  • Any platelet count if patient is bleeding and:
    • Has congenital thrombocytopathy
    • Is taking aspirin or Plavix
    • Has mechanically-induced (cardiac bypass or ECMO) thrombocytopathy

Answer 1 would be correct if the patients would be febrile or septic. For active non-neuroaxial bleeding, platelet transfusions are indicated if the platelet count is <50 x 109/L and for neuro-axial (head, brain, eye, orbit) bleeding if the platelet count is <100 x 109/L.2 Platelet transfusions are also indicated in bleeding patients with congenital, drug (ASA, Plavix)- or mechanically-induced (cardiac bypass or ECMO) thrombocytopathy. In these instances, the platelet count is not relevant, all platelets are affected.3 Answer 3 would be correct if the platelet count would be <50 x 109/L.

Question 4: What are the immune causes for platelet refractoriness (check all that apply):

ABO mismatch between the patient and donor
Rh mismatch between the patient and donor
The presence of HLA Class I antibodies
The presence of HLA Class II antibodies

Explanation to question 4:

The correct answers are A and B.

An appropriate response to platelet transfusion is an increase by ~ 30 x 109/L in platelet count at 10 minutes to one hour after the transfusion of one dose (one apheresis platelet). Platelet refractoriness is defined as the failure to obtain an appropriate corrected count increment (CCI) post-transfusion where CCI is defined as [(post-transfusion platelet count – pre-transfusion platelet count) x body surface area (m2)]/number of platelet transfused (x 1011). The number of platelets in one apheresis platelet bag is 4×1011. An appropriate CCI is >7500. CCI thresholds of <7500 but > 5000 are considered borderline, whereas CCI < 5000 are consistent with platelet refractoriness. Platelet refractoriness may be due to:

  • Non-immune factors, including
    • Bleeding
    • Sepsis
    • DIC
    • Drug effect
    • Other platelet-consumptive process)
  • Immune causes, such as:
    • ABO mismatch
    • Presence of HLA Class I antibodies
    • Presence of antibodies against human platelet antigen (HPA) (Rh antigens/antibodies and the HLA Class II system do not play any role as the antigens in these systems are not present on platelets).

Once platelet refractoriness is assessed, further work-up to identify antibodies against HLA Class I and HPA should be performed to provide antigen-negative platelets that would not be cleared by patient’s antibodies.

Routinely, platelets are transfused across ABO barriers, although transfusion of ABO-matched or compatible platelets is preferred when available. Platelets have A and B antigens on their surface, and these can interact with the pre-formed anti-A and anti-B antibodies. The ABO mismatch becomes more relevant in the context of platelet refractoriness and providing ABO-matched/compatible platelets should be a priority in this context.

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