How does the dexamethasone suppression test work?

Q. I am confused as to why high-dose dexamethasone inhibits a pituitary source, but the lower dose does not. Is it just because the cells are hyperplastic and not functioning up to par? Also I am assuming that dexamethasone produces a metabolite different from those produced from endogenous cortisol in urine, right? Otherwise the test wouldn’t be interpretable.

A. Great questions. Regarding the high vs. low dose dexamethasone and pituitary adenomas: the cells in the adenoma are neoplastic. Benign, but still neoplastic. Neoplastic cells generally are usually pretty insensitive to outside stimuli (but they can, in some cases, be affected). I think of those pituitary adenoma cells as being typical neoplastic cells in that they are insensitive to low-dose dexamethasone.

However, if you give a lot of dexamethasone, well, they do respond a little (and decrease their ACTH production). A similar principle operates in Nelson syndrome: the ACTH-producing pituitary adenoma is somewhat kept in check by the negative feedback from all the cortisol floating around. But if you remove the adrenal glands, you remove the negative feedback, and the pituitary adenoma grows explosively.

Regarding measuring dexamethasone in the urine: dexamethasone is an extremely potent steroid – way more potent than cortisol. So to get the same effect as cortisol, you only need to supply a relatively tiny amount of dexamethasone. The amount used in the suppression test is so tiny that it doesn’t affect the measurement of cortisol in the urine.

Coagulation tests in 500 words or less

First, a quick review of how a blood clot is formed

Here’s how you make a clot: 

  1. Constrict the affected blood vessel
  2. Form a platelet plug
  3. Form fibrin (using the coagulation cascade) to seal up the platelet plug 

If you think back to the basics of the coagulation cascade, you might recall that there are two arms – an extrinsic arm and an intrinsic arm – which come together in the final common pathway, which ends up turning fibrinogen into fibrin. When somebody is bleeding, and you think it’s due to a coagulation problem (as opposed to a platelet problem), it’s helpful to know what part of the cascade is screwed up. That helps you figure out what’s wrong with the patient (is it hemophilia? or liver disease? or coumadin overdose?).

The two main coagulation lab tests

There are two main tests for evaluating the cascade: one for the extrinsic arm (the PT/INR) and one for the intrinsic arm (the PTT). There are other tests too – but those will have to be for another post.

1. The PT/INR

The prothrombin time (PT) is performed by adding thromboplastin to the patient’s plasma, and seeing how long it takes to make fibrin (as soon as you see the first strands of fibrin form, the test is over, and you measure the result in seconds.

Thromboplastin contains a tissue-factor-like substance, and therefore it just measures the extrinsic pathway. The annoying thing about thromboplastin is that its strength varies considerably from manufacturer to manufacturer. So the PT done at one hospital (using manufacturer A’s thromboplastin) will be significantly different than the PT done at another hospital (using manufacturer B’s thromboplastin). Dumb.

To deal with this annoying issue, someone figured out how to make some mathematical calculations that take into account each manufacturer’s thromboplastin strength. Now, you just do your PT assay, add on those mathematical calculations, and you wind up with something called the International Normalized Ratio (INR). This means that you can do the PT with anybody’s thromboplastin, and you’ll wind up with a result (the INR) that removes that variability. Super important. 

2. The PTT

The PTT, or partial thromboplastin time, is performed by adding just some phospholipid to the patient’s plasma and waiting to see how long it takes to form fibrin. You have to add in a little “contact factor” like kaolin to activate XI to XIa – but other than that, that’s it!

It’s called the “partial thromboplastin” time because initially, it was found that by adding a part of thromboplastin to a test tube, you could activate fibrin formation. It turns out that the part of thromboplastin people were adding was just phospholipid, and that thromboplastin consists of both phospholipid and tissue factor. This test measures the intrinsic pathway, which is that arm of the cascade involving factors XI, IX, VIII and the final common pathway.

 

Why does the GFR go down in nephritic syndrome?

Q. I have a question. Why do you see a decreased glomerular filtration rate in nephritic syndrome? I read on your blog and other places that it’s due to “hemodynamic changes.” From Robbins I’m assuming this is compensatory stuff – but wouldn’t that increase GFR?

A. It’s because of what’s going on in the glomerulus! In a normal glomerulus, the capillaries are all nice and open and patent. Blood flows through the capillaries like a little river, fluid gets filtered out into the urinary space, and the GFR is normal. But in nephritic syndrome, the glomeruli are stuffed full of cells, and blood flow slows way down.

Take a look at post-streptococcal glomerulonephritis, a common cause of nephritic syndrome. In that disorder, the glomeruli are huge and hypercellular, with tons of neutrophils in there (and probably some other proliferating glomerular cells as well). The poor capillaries are compressed by all that extra stuff, and you can imagine how hard it is for the poor blood to flow through there! If the blood can’t flow through at the same rate, then the filtration of fluid from blood into urine is decreased (and the GFR slows down to a sad little dribble).

What is an anti-antibody?

Q. I just had a quick question as I was going through some immunology notes and wondering if you could clarify what they mean when a person develops an anti antibody? (more…)

How is iron handled in the body?

Q. I’ve never been clear on the way iron is handled and was hoping you could clarify. Basically, I don’t understand the difference between serum iron and serum ferritin. (more…)

Antiphospholipid antibody syndrome

Q. Can you write a post about antiphospholipid syndrome? I could not find good source which explains its pathophysiology and laboratory results

A. First, before we get into the antiphospholipid syndrome, we need to talk about antiphospholipid antibodies. As you might expect from their name, antiphospholipid antibodies are autoantibodies in the patient’s plasma that are directed against various phospholipids (there are lots of phospholipid surfaces in the body – including the phospholipid surface upon which the coagulation factors interact). There are a bunch of different types of antiphospholipid antibodies, including anticardiolipin antibodies, anti-glycoprotein antibodies, and the so-called lupus anticoagulants (which were discovered in patients with lupus).

In addition to binding to various phospholipid surfaces in the body, these autoantibodies also just happen to bind to the phospholipid part of the PTT reagent (and sometimes, the PT reagent). Then there’s not enough usable reagent in the test tube, and the patient’s specimen doesn’t clot! The coagulation tests are therefore falsely prolonged.

Antiphospholipid antibodies are sometimes called “inhibitors” because they appear to inhibit coagulation in the test tube. But here’s a weird thing: in the body, they can be associated with thrombosis!

You may be asking yourself: how do you get these antiphospholipid antibodies? And are they dangerous?

It turns out there are different answers for different patients. Children, for example, sometimes develop antiphospholipid antibodies after an infection. In this setting, the risk of thrombosis is only slightly increased; it’s usually not a big deal clinically. Adults sometimes develop antiphospholipid antibodies as part of an autoimmune disorder like lupus (in fact, antiphospholipid antibodies – in whatever clinical setting – are often called “lupus inhibitors” because of this association). In these patients, the risk of thrombosis is moderately increased. Finally, elderly adults may develop antiphospholipid antibodies in association with drugs. This is virtually always a harmless event with no increased risk of thrombosis.

Okay, so here’s where we get to the antiphospholipid antibody syndrome part. This term is used when a patient with an antiphospholipid antibody has thromboses or pregnancy-related complications (like recurrent miscarriage, pre-term labor, or pre-eclampsia). This syndrome is a serious thing. In a small number of patients, the thromboses can be widespread, leading to multi-organ system damage and death. The term is reserved for patients who are symptomatic; you wouldn’t use the term in patients who have an antiphospholipid antibody but no symptoms.

So what would you do if you think your patient might have an antiphospholipid antibody? Well, you’d need to confirm this suspicion with laboratory tests. First, order a PTT (in fact, that’s how a lot of these patients get picked up – they present with an abnormally prolonged PTT in the face of clinical evidence of thrombosis).

Then, order up a mixing study. Remember what a mixing study is? You do this test when you have a patient with a prolonged PTT and you want to know why. It’s performed by taking the patient’s (probably abnormal) plasma and mixing it with some pooled (normal) plasma – then running the PTT on this new mixed sample.

If the new PTT value is within the normal range (if it “corrects”), then you know the pooled human plasma must have supplied something to the patient’s plasma to make it clot normally. The “something” is usually a coagulation factor that the patient is missing.

If the new PTT value is still abnormal (if it’s still prolonged, and doesn’t “correct”), then you know that even though you added a bunch of normal plasma to the mix, the patient’s plasma still couldn’t clot normally. There must be some other problem with the patient’s plasma. The “other problem” is usually an inhibitor.

One caveat: some antiphospholipid antibodies do not prolong the PTT. It all depends on the particular PTT reagent your lab is using (some reagents are just more easily swayed by the antiphospholipid antibodies). So if you really feel your patient may have an antiphospholipid antibody, you shouldn’t stop investigating that possibility just because the PTT comes back normal! There are plenty of fancy lab tests that can be done to detect antiphospholipid antibodies. Just call your friendly neighborhood pathologist and see what he/she has to offer.