The Coagulation Conundrum
A 67-year-old male on warfarin therapy is scheduled for a PT/INR check. You are using a 21-gauge butterfly needle with a 12-inch tubing set because the patient has difficult veins. You successfully draw blood into a 3 mL light blue top tube (sodium citrate). The tube fills to the indicated mark. However, you notice that the blood flow was sluggish and the tube took about 30 seconds to fill. The patient's hematocrit is 52% (elevated). You also need to draw a CBC and a chemistry panel. You have a 5 mL lavender top and a 5 mL gold top (SST) ready.
1. What are the potential issues with the PT/INR specimen and how should you proceed?
Read the worked answer
Potential issues: 1. Underfilling of the citrate tube: The tube filled to the mark, but with a butterfly and tubing, there is a 'dead space' of air in the tubing that can cause the tube to underfill if not properly accounted for. However, the tube filled to the mark, so volume may be adequate. 2. Elevated hematocrit (52%): For patients with hematocrit >55%, the citrate concentration may be relatively increased, leading to falsely prolonged clotting times. At 52%, it is borderline; some labs adjust the citrate volume for hematocrit >55%. 3. Sluggish flow: May indicate difficult draw, but the tube filled. 4. Order of draw: The light blue tube should be drawn before the lavender and gold tubes when using a butterfly, but with a butterfly and tubing, a discard tube may be needed to ensure the citrate tube fills correctly. Since you used a butterfly, you should have drawn a discard tube (non-additive) first or used a second light blue tube. You did not do that, so the citrate tube may be short due to the air in the tubing. Recommendation: Recollect the PT/INR using a discard tube or a second light blue tube after discarding the first 1-2 mL. Alternatively, use a 23-gauge needle for the draw if possible. For the CBC and chemistry, proceed with the lavender and gold tubes, but ensure proper order of draw: light blue, then lavender, then gold. Since the patient is on warfarin, the PT/INR is critical; if in doubt, recollect.
2. What is the correct order of draw for the tubes you need, and how does the butterfly affect it?
Read the worked answer
Correct order of draw for venipuncture: 1. Light blue (sodium citrate) for PT/INR, 2. Lavender (EDTA) for CBC, 3. Gold (SST) for chemistry. When using a butterfly needle with tubing, the order of draw is the same, but a discard tube (non-additive or a light blue tube) must be drawn first to clear the air from the tubing. This discard tube is not used for testing. Then draw the light blue tube for PT/INR, followed by lavender, then gold. If a discard tube is not used, the first tube may be short due to air in the tubing. In this case, since the PT/INR is critical, it is best to recollect with a discard tube to ensure accurate results.