Why is the hemoglobin normal right after a big blood loss?

blood-drop

Q. Immediately after an acute episode of blood loss – following a motor vehicle accident, for example – the hemoglobin level is normal. Why is that?

A. It’s true: immediately after acute blood loss, the hemoglobin is indeed normal!

This might seem counterintuitive at first. Shouldn’t the hemoglobin be decreased since there’s less blood in the patient?

But if you think about this a bit more, during acute blood loss, you’re losing not just red cells but also every other blood component (including plasma). So the blood remaining in the patient at that point is totally normal – it’s just that there isn’t enough of it.

This means that if you take a sample of the patient’s blood right after a big blood loss, it will look normal. It has a normal number of red cells per unit volume, and the red cells themselves are perfectly normal (assuming the patient’s blood was normal to begin with).

After a few hours (sooner, if you give the patient fluids), the blood will start to become more dilute as the patient pulls fluid from tissues into vessels. If you measure the hemoglobin (and the RBC) at this point, both will now appear decreased – and rightly so, because the total blood volume has now increased.

The RDW, MCH, and MCHC, by the way, will be normal even at this point – because these tests measure the variation in size of the red cells (in the case of the RDW) and the amount of hemoglobin in each red cell (in the case of the MCH and MCHC). The patient’s problem is that there are not enough red cells around. The ones that are there, though, are completely normal.

How to read a bone marrow biopsy

normal marrow

There are several things you should look for when evaluating a bone marrow biopsy specimen – see if you can see them in the image above.

First, take a look at the cellularity. The white spaces are fat cells that have washed out during processing; the cells in between the fat cells are hematopoietic precursors. The ratio of cells to fat is called the “cellularity.” The marrow above is approximately 30-40% cellular. You need to know the age of the patient to estimate whether the cellularity is normal. Here is a rough guide to cellularity by age:

0-3 months: 100%
3 months – 10 years: 80%
20 years: 65%
30 years: 50%
40 years: 45%
50 years: 40%
60 years: 35%
70 years and over: about 30%

Next, take a look at the composition of the marrow. Myeloid cells make up the largest percentage of the normal marrow cellularity; erythroid cells are second most common. The ratio of myeloid to erythroid cells should be about 2:1 to 4:1. It’s easier to see these cells on an aspirate smear, but you can get a pretty good idea on the marrow section too. Neutrophils and precursors often have eosinophilic, granular cytoplasm; if you look closely, you can see the indented nuclei of metamyelocytes and segmented nuclei of mature neutrophils. Erythroblasts generally have very round, dark nuclei; earlier forms are large, and later forms are small. A few megakaryocytes (large cells with abundant eosinophilic cytoplasm and multiple nuclei) should be sprinkled throughout the marrow too. Lymphocytes normally represent about 10-15% of the marrow cellularity. The above marrow appears to have a myeloid:erythroid ratio of 2:1, and megakaryocytes are normal in number.

Finally, take a look through all the sections to see if you see anything weird, like fibrosis, metastatic carcinoma, lymphoid aggregates, or amyloid deposition. As you scan the sections, you should see evenly-distributed cellularity, with evenly-spread hematopoietic precursors. Anything that deviates from this pattern should be investigated on higher power.

Metaplasia vs. neoplasia

adenocarcinoma

Q. My professor asked this on an exam: What’s the difference in molecular mechanism between metaplasia and neoplasia?

A. Metaplasia is the changing of one cell type to another. The term is used most often in reference to epithelial cells, for example, when the normal glandular epithelium of the cervix is replaced with squamous epithelium, it is called “squamous metaplasia”. It simply means that the basal cells (the stem cells of the epithelial layer) switch from making one type of epithelial cell to another.

Though it is not malignant or even premalignant, in and of itself, metaplasia sometimes indicates that there has been damage to the area, and if the insult continues, dysplasia or even frank malignancy can occur. This is fairly common in the lung: metaplasia of the bronchial epithelium is followed by dysplasia, which is followed by carcinoma. The molecular mechanisms of this whole process of metaplasia are not well understood.

“Neoplasia” literally means “new growth.” Neoplastic cells have several characteristics that make them nasty: they grow autonomously without any need for growth signals, they are insensitive to normal growth-inhibitory signals, they don’t die off like they should, they are capable of limitless replication, and – if they are malignant neoplastic cells – they invade vessels and travel to different parts of the body and set up shop.

There are lots of molecular mechanisms (and corresponding genetic mutations) that underlie these neoplastic qualities; most neoplasms have several such mutations. A cancer cell can have mutations in many different genes – for example, the genes encoding growth factor receptors, signal-transducing proteins, nuclear transcription factors, or cyclins.

Sometimes these mutations turn on a gene that promotes growth. The normal variants of these growth-promoting genes are called “proto-oncogenes” and the mutated variants are called “oncogenes.” An example of just such a gene is the RAS proto-oncogene, which makes a signal transduction protein involved in cell growth. Many neoplasms have a mutated RAS gene (called the RAS oncogene) that has been altered in such a way that it is always turned on. Which means that the cells containing the mutation are always transducing growth signals, and always growing and dividing.

Another type of mutation can occur in genes (called “tumor suppressor genes”) that normally put brakes on cell growth. An example of this type of gene is the retinoblastoma tumor-suppressor gene, which normally stops cells at the G1 checkpoint in the cell cycle. In certain tumors, the retinoblastoma gene is mutated in such a way that it doesn’t work. Cells that have this mutated gene proceed without pause through the G1 checkpoint, heading full-tilt on to mitosis.

So, to summarize: the molecular mechanisms of metaplasia are not well understood. The molecular mechanisms underlying neoplasia are numerous and complex.

What does the bleeding time really measure?

white-bleeding-heart

Here’s a very good question about the diagnostic use of the bleeding time.

Q. I’m currently studying heme for boards and came across a practice questions that used platelet count, bleeding time, PT and PTT values to differentiate between certain diseases/problems. I was just wondering how in both Vitamin K deficiency and liver disease you can get an increase in PT and PTT but the bleeding time doesn’t change…I guess I figured that bleeding time would have to increase.  Can you explain this to me?

A. Yeah, that does sound weird, you’d think the bleeding time would change – but actually, the bleeding time is only a measure of platelet function. It really has nothing to do with coagulation!

I kind of think of it like this: the platelet plug is the first thing to form, and that is enough to stop the bleeding from the incision made at the beginning of the test. The coagulation cascade happens next, and the status of that won’t be apparent in the bleeding time results. The patient might have some more bleeding later if their coagulation system is really screwed up…but the bleeding time assay will be done by then. In reality, it probably happens a little more concurrently than that (platelet plug is followed very closely by fibrin formation – the two probably even overlap a bit), but I think it’s a good way to remember the concept.

The same reasoning fits with the way that people with coagulation factor disorders bleed (as opposed to patients with platelet disorders). People with platelet abnormalities tend to bleed spontaneously into mucous membranes without much provocation (probably because they’re having a hard time forming that initial platelet plug) whereas patients with coagulation factor abnormalities, like hemophilia, tend to have deep, severe bleeds that happen after some time has elapsed (because they form the initial platelet plug okay, but they can’t seal it up with fibrin very well, so they end up bleeding later on).