How to differentiate between true and false hyponatremia

In this video from our Fluids and Electrolytes Masterclass, you will become a pro at differentiating between true and false (or factitious) hyponatremia!

Joel Topf, MD
Joel Topf, MD
3rd Oct 2019 • 4m read

True and false hyponatremia can be easily confused—if you don't know what you're looking for! In this short video, nephrology mastermind, Dr Joel Topf, explains how to differentiate between true hyponatremia and false hyponatremias (factitious hyponatremia and pseudohyponatremia). You'll understand why false hyponatremias show up in lab reports and how you can adjust the numbers to figure out what's really going on with your patient.

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Video Transcript

[00:00:00] The true hyponatremia is when you have a low serum sodium concentration, associated with low osmolality and low tonicity and it will cause water to move into the cells, as they move into the relatively hypertonic intracellular compartment. Water shifts into the cells and cause them to stretch and that will cause symptoms. False hyponatremia is when you have a low serum sodium concentration

[00:00:30] but the actual cellular osmolality or tonicity is normal or high, okay? And so, if it's normal, you'll get no movement, if the extracellular tonicity is even higher than the intracellular tonicity, you'll get movement of water out of the cells. But in false hyponatremia, water does not flow into the cells and that's what we expect to happen with hyponatremia, that's why we call it false hyponatremia. There are two varieties of false hyponatremia.

[00:01:00] In one, the water does not move at all, we call that pseudohyponatremia. And then the other, the water moves out of the cells and we call this factitious hyponatremia. Let's start with pseudohyponatremia. In this one, the osmolality and tonicity are normal. And the reason you get hyponatremia in the lab is just a lab measurement error. This is a problem with the instrument, that's used to measure the sodium, that fails in a specific clinical

[00:01:30] scenario when there are an unexpectedly high number of immunoglobulins or an unexpectedly high amount of lipids. So, what happens with this lab measurement equipment is they will report the sodium content divided by the plasma water, but they'll actually measure the sodium content divided by the plasma volume. Normally, these are very similar, maybe off by about 10% and they put a little bit of a factor, an adjustment

[00:02:00] factor so when they measure this and report, this is adjusted out to be normal. But if you have an unexpectedly high insoluble fraction, unexpectedly high proteins in the blood or unexpectedly high lipids in the blood, all of a sudden your plasma water is much smaller than your plasma volume. And when they measure sodium over plasma volume, it wildly underestimates the true sodium concentration because

[00:02:30] the denominator is so much higher than reality. We see pseudohyponatremia with increased lipids, so hyperlipidemia and increased proteins. Usually, this is immunoglobulin and you'll see this in multiple myeloma or Waldenstrom's macroglobulinemia or with the therapy IVIG, which is what's here. Those patients will get a pseudohyponatremia.

[00:03:00] Now, not every lab instrument is susceptible to this error. Lab equipment that has direction ion detection are not susceptible for this error and you can just get on the phone and call your pathologist at your hospital and find out what method they're using to measure sodium and see if it's susceptible to high lipids or high proteins, if you suspect this is going on. But the best way to detect it, if you get hyponatremia, order a serum osmolality and see if it's low. If it's low, you're dealing with true hyponatremia. If it's high or normal,

[00:03:30] you're dealing with some form of false hyponatremia. The other form is called factitious hyponatremia. And here, instead of water moving into the cells, like we'd expect with hyponatremia, water is actually moving out of the cells. Let's take a look at what's going on here. So, here's a patient with an extracellular hypertonic environment. Hypertonic concentration much higher than the intracellular concentration and that's going to draw water osmotically out of the cells. You get osmotic movement of water,

[00:04:00] out of the cells. And this water, that's moved out of the cells, is going to dilute the serum sodium out here, and that's going to cause the low sodium. So, the low sodium is true, not that there is a measurement error. The sodium is actually low. It just doesn't have the implications of normal hyponatremia because the water is moving in the opposite direction, out of the cells rather than into the cells. We see this in hyperglycemia. This is pretty common, some people can get hyperglycemia. Mannitol is used to shift water out of the cells to lower

[00:04:30] intracranial pressure. And then glycine is a dilutant that's used in bladder surgery and prostate surgery and it sometimes will accidentally get into the blood and causes same type of picture. Hyperglycemia, in the presence of insulin, doesn't move water. Glucose, in the presence of insulin, is an ineffective osmol. And so, if there's plenty of insulin, this hyperglycemia is just going to move into the cells under the influence of insulin and it's not going to cause a shift of water out. One of the interesting things

[00:05:00] about factitious hyponatremia or hyponatremia due to hyperglycemia is that the degree of hyponatremia is predictable. You can figure out what the serum sodium would be if the hyperglycemia was gone. So, suppose you had a patient with a sodium of 128 and a blood sugar of 400, what you do, is that for every 100 that the blood sugar is above 100, you add 1.6 to the measure of sodium. So, with 400 we have three 100s above

[00:05:30] 100, 3 times 1.6 is 4.8, 4.8 plus 128 is 133. So, this sodium of 128 really represents a sodium of 133, if the glucose was normal. Okay, that's called the adjusted sodium. Some people like to use 2.4 rather than 1.6. In my experience, I think 1.6 is more accurate but your mileage may vary.