Phosphorus (PO4) Blood Test Lab Values

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Determine the significance and clinical use of measuring Phosphate in clinical practice


Lab Test Name:

Phosphorus/Phosphate PO4



Lab taken to measure phosphate concentration of the serum

Involved in development of:

  • Bones and teeth
  • Cell membrane
  • Nucleic acid
  • Adenosine Triphosphate (ATP)
  • Intracellular signaling proteins



Drawn to evaluate/diagnose:

  • Renal disease
  • Parathyroid disorder

 S/S include:

  • Fatigue
  • Muscle cramps
  • Bone pain

Normal Therapeutic Values:


  • 3.0-4.5 mg/dL


  • Plasma separator tube


What would cause increased levels?

Increased: >4.5 mg/dL

  • Acute overload- over ingestion
    • Repletion
    • Phosphorus -containing laxatives
  • Vitamin D toxicity
  • Cellular lysis
  • Decreased urinary excretion
    • Kidney failure
    • Tubule disease
  • Intracellular→extracellular
    • DKA- no insulin available, so phosphate leaves the cell to the extracellular space


What would cause decreased levels?

Decreased level: <2.5 mg/dL

  • Hyperparathyroidism
  • Decreased intake
    • intestinal absorption, malnutrition, chronic alcoholism
  • Vitamin D deficiency
  • Increased urinary excretion of phosphate
  • Extracellular→ intracellular
    • Refeeding syndrome- native insulin release
    • Insulin administration- DKA
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Hey everyone, Abby here with This lesson covers the lab value for phosphorus. We’ll talk about the normal lab value and conditions in the clinical picture that would cause increased and decreased values. Let’s dive in. 


Phosphorus or phosphate lab abbreviated as PO4 is a lab that’s taken to measure phosphate concentration in the serum. Now, why are these both here? Phosphorus is a mineral. So, let’s talk about the difference between these two. Phosphorus is a mineral and phosphate is an electrolyte. So, in our bodies, this is the part that actually has an electrical charge and does something in our tissues, and it contains the mineral phosphorus. This is a very important nutrient in the development of very important tissues. As you can see here, the bones and teeth,  cellular membranes, nucleic acid, and even ATP. Have you ever noticed the tri phosphate part of ATP? It’s also used in the development of intracellular signaling proteins. So, very important. Now, can you think of where we might be concerned with phosphorus, or phosphate, or where it might be excreted? We use this lab when we’re evaluating or diagnosing renal disease. The nephron is very much involved in keeping a balance in our electrolytes, in the serum, and the kidneys are responsible for filtering excess phosphate from the blood. When lab values are outside of normal, that can indicate kidney disease. It can also indicate parathyroid disorder. When there’s a decrease in serum calcium, there’s an increase in phosphorus. And what that does. is it causes the parathyroid to release parathyroid hormone. What does parathyroid hormone do? It asks the bones to release calcium, to increase the serum calcium concentration.  Signs and symptoms when one of these conditions is apparent, are fatigue, muscle cramps, and even bone pain. 


Normal therapeutic values are between 3 and 4.5 milligrams per deciliter. When I worked in the CVICU the cardiovascular ICU, I had patients on CRRT all the time, continuous renal replacement therapy, and we would draw a phosphorus lab every morning. We wanted to know how much that CRT had chewed up the phosphorus or the phosphate and if we needed to replete. Collection for, uh, measuring the phosphate is done in a plasma separator tube, like this green tube here. When lab values are increased, that’s a level that’s above 4.5 milligrams per deciliter. It’s typically related to an acute overload, so repletion might have been too much, or this could be a patient that’s taking too many phosphorus-containing laxatives. It’s also apparent in vitamin D toxicity and when cells lyse. They’re gonna release the phosphate, right? And, we talked about renal failure and when there’s issues in the nephron and the tubal specifically, that they’re not excreting the phosphorus, that’s going to really increase the lab value. 

Now, something that’s super interesting is that normally the phosphate should be inside the cell. It becomes extra cellular in cases like DKA, because there’s not enough insulin to drive the phosphorus into the cell, along with the glucose. So, pretty interesting relationship there. 


This lab will be decreased or below 2.5 milligrams per deciliter in hyperparathyroidism. If we have too much PTH, that’s going to talk to the calcium and phosphorus, right? If there’s a decreased value, it could also be that there’s a decreased absorption or malnutrition and even in the chance of chronic alcoholism. Vitamin D deficiency is also a time when we would see a decrease in the phosphate. Now, if there’s increased urinary excretion, of course that’s going to decrease our phosphate and then, the inverse of what we just talked about with the extracellular movement to intracellular, is when someone is suffering from refeeding syndrome. So, in refeeding syndrome, the patient, all of a sudden has all of these nutrients, and there’s a large amount of insulin release and the insulin drives D, phosphate, and glucose into the cell. So, it moves extracellularly to intracellular and then when it’s in the cell, we see a decreased lab value. It’s also going to be apparent when correcting diabetic ketoacidosis. When insulin is involved, we’re going to have a shift of the phosphate and glucose into the cell.


Linchpins for this lesson are that the phosphate lab, the PO4, measures the electrolyte amount in the serum. Phosphorus is the mineral remember, and phosphate is the electrolyte. Normal value is between 2.5 and 4.5 milligrams per deciliter. If the value is increased, that means it’s above 4.5 milligrams per deciliter, and indicates an over ingestion of the electrolyte, or a lack of excretion.  Decreased value would be below 2.5 milligrams per deciliter, and indicates either an inadequate intake, or increased excretion too much is being filtered. 


You all did great on this lesson. Now go out, be your best self today. Remember, we’re always behind you, and happy nursing.


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