Base Excess & Deficit

The last lab value were going to talk about when it comes to arterial blood gas results is the base excess and base deficit. This is actually one lab value that technically goes by two names.

The official definition of base excess is the level of excess or possibly deficiency of base or alkaline substances present in the blood. The normal value is -2 to +2 mEq/L. so, if the number is negative, it is actually referred to as a base deficit. If the number is positive it is referred to as a base excess. So, while base deficit and base access refer to the same lab result, they are not the same thing. Most people will use base excess, but then tell you that the number is negative. This is not completely wrong, but it’s important that you understand what I mean when I say a base excess versus a base deficit.

The meaning of the base excess value is that it is a strong indicator of the metabolic component of acid-base balance. A high positive level, in other words a level greater than positive 2, indicates metabolic alkalosis. This means we have extra base in our blood. Most commonly this is caused by an excess of bicarb in the blood. If we have a low negative level, in other words a level less than -2, this indicates metabolic acidosis. This means we have a deficiency of base in our blood, or a state of acidosis. Remember that a metabolic acidosis could be caused by too many acids or by not enough base. If we see an anion gap, we know that there are extra acids floating around in our blood that we aren’t able to measure, therefore that is the likely cause. If we don’t have an anion gap, there is a good chance that this acidosis is caused by a loss of base, such as in diarrhea.

While the base excess is a strong indicator of the metabolic component, it does have some limitations that make it a little bit less reliable. One of those limitations is in any patient that has some sort of underlying or chronic illness that causes acid base disturbances. If I have a client with renal failure who lives in a state of metabolic alkalosis, they may have a high base excess all the time. Then, you see their base excess drop to 0 and think they’re perfect, when in fact they actually are having some sort of acidosis process on top of their chronic alkalosis. In somebody with COPD who may live in a respiratory acidosis state, I may not notice a change in their base deficit, or I may see a base deficit and assume they are septic when actually that’s where they live all the time. The other thing that can create a falsely High base excess is fluid resuscitation. The best way to overcome any of these limitations is to look for Trends in the clients face access numbers. If somebody has a base excess of 6 and the next day a Base excess of 2, which is normal, and the next day a base deficit of -1, which is also normal. If we are just looking at the individual values, we may not see the red flags of this patient trending towards acidosis.

So it is extremely important to evaluate the entire blood gas as a whole and to look for these Trends. None of the values that we’ve talked about in this course should ever be assessed in isolation. We should always consider the big picture of what’s really going on with our patient. We can also take the values we obtained from our blood gas and compare them to the other clinical findings in our patient to understand what is causing the problem and how they would benefit the most from various treatments. For example looking at the P/F Ratio or the anion gap to decide whether these individual values are good or bad for your specific patient.

So, let’s recap. Remember that face access is all about the extra or deficiency of Base in our bloodstream. Extra-base would give you a high positive and means alkalosis and a deficiency of base or a base deficit, which is a low negative number means acidosis. Make sure that you are looking at the trends in your patient’s lab value because chronic illness or other interventions might change the patient’s Baseline. And, as always, look at the entire ABG of whole as well as comparing it to other lab values to get the big picture of what’s really going on with your patient.

So that is it for base excess and base deficit, and our entire ABG course. Make sure you check out all the resources attached to this lesson, and don’t hesitate to go back through any of the lessons in this course that you need clarity on. Now, go out and be your best selves today. And, as always, happy nursing!!