Monday, March 2, 2009
What does that test result mean?
What does that test result mean?
Coagulation is a puzzle; you need all of the pieces to make a complete picture.
That is a difficult task in the laboratory - you need the test result, the patient history, what type of therapy or anticoagulant they are on, or off of, or being transitioned on too-.Let's face it that never happens in the laboratory- you are lucky to get anticoagulant information. So you have this test result, is it normal, is it abnormal- well it depends on your patient and the situation. What might be normal for one patient may be abnormal for the next.
Patient goes to her OB/GYN, she is pregnant, and complains of shortness of breath. This is her third pregnancy. Pregnancy unbalances the haemostatic scale, and can place you at a risk for a bleed or thrombosis. Compounding this with a hereditary predisposition, makes a pregnant women a ticking timebomb. Knowing this along with the symptom of shortness of breath, the OB orders several blood tests, in particular a thrompophilic workup. The results are as follows:
Protein C = 89% (80-120%)
ATIII = 107% (70-130%)
APCR = 2.5 (>2.1)
Lupus = negative
Protein S = 34% (65-135%)
He informs the patient they are Protein S deficient, and both the baby and the mother are at risk. So what are the pieces to this puzzle? This is her third pregnancy, she has not had any previous event, it would be unusual for this to not have been manifested previously- so what does this test result mean?
This is a normal test result for pregnant women. Her thrombophilia work-up is normal for being pregnant. Protein S is decreased in pregnant women, contributing to their hypercoagulable state. So this abnormal result is normal for this patient. Her shortness of breath is just an occurrence of pregnancy. If the clinician had acted on this result, the outcome could have been disastrous.
Okay, next result; we have a mom in labor and delivery they have sent down three fibrinogen levels on this patient. All results are in the normal range-
We have a 289 mg/dL, 250 mg/dL and 245 mg/dL. They have all been sent in the last six hours, how many do they intend to send? They are all normal, correct? Well not exactly - what are the pieces to this puzzle? .Fibrinogen is an acute phase reactant; this means this factor is elevated in times of inflammation, stress, trauma and pregnancy. One would expect this result to be elevated in a pregnant women, this result is in the normal range (150-450,000). What does this mean? It means that this might be being consumed, as in disseminated intravascular coagulation (DIC). So this normal result, is not normal at all for a pregnant women. This result needs to be acted on to prevent the progression of DIC- which is the simultaneous formation of thrombin and plasmin. This condition has a high degree of mortality for both the mother and the fetus. So a normal fibrinogen in this patient is not normal at all.
What about when a clinician wants to add on an APTT to an existing PT sample. You know the routine, the sample arrives in the lab with the morning bloods, and a PT is ordered. They really needed an APTT so they call 6 hours later to add it on. Now we know that an APTT is only stable for 4 hours. This was a pediatric draw, the result is important and the sample was difficult to obtain. So you run it off line, and it is 35.1 sec (27-37.5 sec). It is normal, so what is the harm, you give it to the doctor. That was so nice of you! However, the problem is that, this pediatric patient was on heparin. The result should have been about 55 seconds. So now what? Well based on the result from the laboratory, the patient needs to have their heparin increased. Remember this is a pediatric patient, but the treatment is based on a result that is 4 hours old. Why is the APTT stable for only 4 hours, because when plasma sits on red cells, the PF4 from the platelets neutralize heparin, if you don't believe this, run a heparinized sample from a previous shift and compare the results. So, as no good deed goes unpunished, your "normal" result, now put that patient at an increased risk for a bleed- most likely his level APTT reflected an appropriate level of heparin, and based on this result, the clinician may increase it- not good. Be careful, stick to the guidelines- they are there for a reason. And as we know, sometimes normal is not the correct answer.
What about the anti-Xa assay that comes out to be 0 U/ml? What is that, and what does it mean?- Your controls are normal - so that means your reagents are okay right? What if you are not sure? Run a previous patient, or a CAP sample with known results, that will validate that your assay is working. (Keep all proficiency samples, and you should keep a stock of patient results, to check your assays). But you still have a 0, and the clinician is telling you that is impossible. How can you determine if there is heparin in that sample? The best test for residual heparin is a thrombin time- it should be prolonged with both unfractionated and low molecular weight heparin. If that comes out normal, there is no heparin in that sample. Either the sample wasn't drawn at the correct time, or there is no heparin on board. Find out how long after heparin was given, was the blood sample taken. A repeat sample may give more accurate results.
So, based on the limited information that we get in the laboratory- it is important to understand what test results mean - or better yet, what they can mean. The more pieces to the puzzle you have, the clearer the picture becomes.
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