Phosphorus Repletion: Products and their Electrolytes

Monitoring and balancing electrolytes seems to be an inevitable daily function of physicians and other healthcare professionals in the internal medicine and intensive care settings.  Sodium, potassium, magnesium, calcium, and phosphorus seem to constantly need attention when a patient is ill in the hospital (and also because you're checking them every day, sometimes with and sometimes without good reason).  

 

Hypophosphatemia background

Hypophosphatemia is common and the causes are many, including (just like other electrolytes): (1) reduced intake, (2) increased elimination, and (3) shift to a different compartment (there are typically 100 molecules of phosphate inside the cell for every 1 extracellular molecule so slight shifts can make a large difference, considering we generally measure the extracellular amounts).  This last point implies that serum concentrations may not reflect total body stores very well which is relevant when deciding whether or not to replete low phosphorus and, if so, what dose to use.

Patients are often lacking in symptoms when hypophosphatemia is mild.  Also, their symptoms may not be recognized as due to hypophosphatemia because the most common symptom is muscle weakness, a vague and general complaint that could be countless conditions.  Severe hypophosphatemia can have neurologic symptoms such as altered mental status, irritability, seizures, numbness, and cardiac/respiratory failure, all of which, of course, also have many causes and none of which clearly point to serum phosphorus concentrations.

Phosphate is an electrolyte with important roles in many cellular functions.  Based off of the symptoms, it is reasonable to connect that it has an important role as an energy source (think about all of the muscles involved in the previously mentioned symptoms and recall the "P" in "ATP" is phosphate). It is regulated by a number of systems, namely gastrointestinal (for absorption), renal (for elimination), bones (for storage), and parathyroid/vitamin D/FGF-23 (regulating compartment transitions). Derangements or diseases in any of these systems can affect phosphorus concentrations in the blood and rates of hypophosphatemia are higher in patients with diabetic ketoacidosis, malnourishment, and sepsis. 

Medications

Medications can also cause hypophosphatemia and here is a list of some of the more commonly implicated medications (not all inclusive):

• Diuretics, aminoglycosides, TK inhibitors, mTOR inhibitors, bisphosphonates, denosumab, ibuprofen, valproic acid, various antiretrovirals, ethanol, intravenous iron, corticosteroids, insulin, parenteral nutrition
  

 

Repleting phosphorus - general

Repleting phosphorus can be confusing and if done incorrectly, can result in significant harm. There are a variety of products to choose from, both oral and intravenous, and each form has its considerations and drawbacks.  Choosing a dose (or how many mmol of phosphate to give) depends on the clinical scenario - as in, what the etiology of the hypophosphatemia is and can it be corrected, the serum phosphorus concentration, severity of symptoms (if any), and renal function, among others.  One weight-based dosing equation can be found here (very easy to use).  These dosing decisions, plus the sometimes controversial decision on whether phosphorus should be repleted at all, will not be covered here, but we will discuss picking a medication.

 

Serum phosphorus severity

While the range definitions for hypophosphatemia vary depending on source ^{Ref, Ref, Ref} ranges are generally defined as the following:

• Normal serum phosphorus: 2.5-4.5 mg/dL
• Severe hypophosphatemia: <1 mg/dL
• Mild/moderate: this one is less clear 
• Some sources have moderate as 1-2.5 mg/dL and 
• Mild extending into the low end of the reference range (e.g., 2.5-3.5 mg/dL)

 

Repleting phosphorus - details

If deciding to replete phosphate, one must first decide between oral versus intravenous routes of administration.  Oral administration is typically used for mild to moderate hypophosphatemia whereas intravenous administration is used in severe symptoms or severe hypophosphatemia (<1 mg/dL). Intravenous should also be used when the patient cannot tolerate the oral formulations.

 

One of the issues with repleting phosphorus is you can not just give "phosphorus" alone.  Phosphorus is the highly reactive element/atom we see on the periodic table.  In medication form, it is given as the phosphate (PO4^3-) ion, in combination with either sodium or potassium.  So, unfortunately, in order to give someone phosphorus, you also have to give them one of these other electrolytes (or both) and the amount in the various products can be surprisingly high.  The following table includes some of the more popular products that are currently on the market and the amount of phosphate, sodium, and potassium in a single dose of each and then what the patient might get with a typical repletion dosing regimen.  Recall, again, that whether to give phosphate and, if so, how much to give, is a separate discussion and depends on many factors.

Phosphate-containing products

So looking at this table you can see there are different amounts of sodium and potassium in every orally administered product and that some of the differences could be significant.  For example, if one chooses Phos-NaK and gives the typical four doses per day, their patient will also get almost 30 mEq of potassium in addition to the phosphate.  This could be harmful if the patient were hyperkalemic or you were trying to restrict potassium intake.  On the contrary, giving four doses of K-Phos Neutral will yield a relatively small amount of potassium (4.4 mEq) but a potentially problematic amount of sodium (52 mEq).  For some perspective, there are 17 mEq of sodium in one tablet of sodium chloride 1 g so giving 4 tablets of K-Phos Neutral is like giving sodium chloride tablets 1 g three times.  This could of course be problematic in a patient that is on a sodium restricted diet or has a disease state that tends to retain fluid.

The same situation exists with the intravenous formulations but these only contain one of the two salts so you have to decide what you would rather have with your phosphate.  Some additional logistical points about the intravenous formulations are that the doses in the table are the readily available products and the pharmacy department may have protocols or be unwilling to make à la carte-type doses (i.e., you might use a weight based calculator to decide you need 22.5 mmol of phosphate but that might not be on option based on institutional protocols).  This could be annoying but keep in mind that these types of policies often exist to reduce errors.  Intravenous phosphate needs to be diluted and ran over several hours (usually 4-6 hours) to prevent toxicities such as infusion-related thrombophlebitis and calcium-phosphate precipitation.  Infusion pumps needs to be set up and nursing educated so limiting the doses available, diluent type, diluent size, and rate of administration all may help reduce errors.

Take home pearls:

• You cannot just give "phosphorus", it is always in salt form with either sodium, potassium, or both.
  
• There are many phosphate repletion products available and they are not interchangeable, the amount of electrolytes in each varies and can be seen in the above table.
• There can be a significant amount of non-phosphate electrolytes in your phosphate product so keep them all in mind when balancing electrolytes and fluid.
  
• First, decide if you are going to replete or not. Then, pick a dose based on mmol of phosphate. Last, decide which electrolyte you want more or less of and pick the best product.
• You should probably order your intravenous phosphate repletion in multiples of 15 mmol as that is what is available commercially - institutional protocols may limit you to this number anyway.
  

 

References:

Amanzadeh J, Reilly RF Jr. Hypophosphatemia: an evidence-based approach to its clinical consequences and management. Nat Clin Pract Nephrol. 2006 Mar;2(3):136-48. doi: 10.1038/ncpneph0124. PMID: 16932412.

Iheagwara OS, Ing TS, Kjellstrand CM, Lew SQ. Phosphorus, phosphorous, and phosphate. Hemodial Int. 2013 Oct;17(4):479-82. doi: 10.1111/hdi.12010. Epub 2012 Dec 20. PMID: 23279081.

Tebben PJ. Hypophosphatemia: A Practical Guide to Evaluation and Management. Endocr Pract. 2022 Oct;28(10):1091-1099. doi: 10.1016/j.eprac.2022.07.005. Epub 2022 Aug 6. PMID: 35940468.

Sharma S, Hashmi MF, Kaur J, et al. Hypophosphatemia. [Updated 2024 Feb 12]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK493172/

photo by fdecomite (no changes made)