TANYA'S

COMPREHENSIVE GUIDE TO

FELINE CHRONIC KIDNEY DISEASE

 

 

THE IMPORTANCE OF PHOSPHORUS CONTROL

 

ON THIS PAGE:


Why High Phosphorus Levels Matter


Symptoms of High Phosphorus Levels


Measuring Phosphorus and Target Blood Phosphorus Levels


Summary of Phosphorus Goals


Ways to Control Phosphorus Levels


 

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Home > Key Issues > Phosphorus

 


Overview


  • If your cat's phosphorus level (in blood tests) is over 6 mg/dl (USA) or over 1.9 mmol/L (international), it is too high and you need to get it under control.

  • Keeping phosphorus levels under control should help slow the progression of the CKD, reduce the risk of serious problems (including heart problems) and make your cat feel better.

  • Feeding a food low in phosphorus is the first and best step. Ideally you want a food with a level below 0.5% phosphorus on a dry matter analysis basis. Prescription kidney foods are the only complete foods which meet this criterion.

  • If your cat will not eat the prescription diet, feeding a food as low in phosphorus as possible and adding a phosphorus binder to the food when appropriate can help control your cat's phosphorus levels.

  • On the left is a list of this page's contents.

  • Please also visit the Phosphorus Binders page.


Why High Phosphorus Levels Matter                                                               Back to Page Index


 

Phosphorus is a mineral essential for good health which is contained in many foods. The body is very good at regulating its phosphorus levels by removing excess phosphorus via the kidneys. However, the kidneys of a CKD cat can no longer efficiently excrete excess phosphorus, so the vast majority of CKD cats will develop levels of phosphorus in their blood which are too high: this is known as hyperphosphataemia.

 

In contrast to the protein debate, there is no dispute about the importance of treating hyperphosphataemia because of the problems it causes. High phosphorus levels:

 

Make the Cat Feel Unwell


High phosphorus levels can cause a variety of unpleasant symptoms and generally make a CKD cat feel quite unwell. There is a list of possible symptoms here.

 

Cause CKD Progression


High phosphorus levels can make CKD progress more quickly. Serum phosphate and mortality in patients with chronic kidney disease (2010) Eddington H, Hoefield R, Sinha S, Chrysochou C, Lane B, Foley RN, Hegarty J, New J, O'Donoghue DJ, Middleton RJ & Kalra PA Clinical Journal of the American Society of Nephrology 5(12) pp2251-2257 found that the risk of dying in human patients in stages 3 and 4 was higher in patients with higher phosphorus levels, even if the phosphorus was within normal range. A study of cats, Survival in cats with naturally occurring chronic kidney disease (2000-2002) (2008) Boyd LM, Langston C, Thompson K, Zivin K & Imanishi M Journal of Veterinary Internal Medicine 22(5) pp1111-7, found that there was an 11.8% increase in the risk of death for every one mg/dl increase in phosphorus in the cat's blood.

 

Survival of cats with naturally occurring chronic renal failure: effect of dietary management (2000) Elliott J, Rawlings JM, Markwell PJ, Barber PJ Journal of Small Animal Practice 41(6) pp235-242, found that the cats who ate reduced phosphorus food or food with added phosphorus binders lived more than twice as long as those who did not.

 

Cause Secondary Hyperparathyroidism


Elevated phosphorus levels can adversely affect calcium levels and eventually can trigger problems with a hormone called parathyroid hormone (PTH), causing a serious condition called secondary hyperparathyroidism (which is not the same thing as hyperthyroidism, although Cornell University College of Veterinary Medicine does mention that around 20% of cats with hyperthyroidism also have elevated phosphorus levels).

 

In Chronic kidney disease (CKD) in dogs and cats - staging and management strategies (2015) A Presentation to the Virginia Veterinary Medical Association 2015 Virginia Veterinary Conference, Dr D Chew states that it is possible for a CKD cat to develop secondary hyperparathyroidism even if phosphorus levels and ionised calcium levels are normal. He explains "In the early stages of chronic kidney disease increased levels of parathyroid hormone (PTH) keep serum phosphorous within the normal range by increasing phosphate excretion into urine. This allows for normalization of serum phosphorous at the expense of hyperparathyroidism." Therefore, even a cat with normal phosphorus levels but with elevated PTH levels would benefit from phosphorus restriction.

 

May Reduce Response to Anaemia Treatment


High phosphorus levels may reduce the cat's response to ESAs, a treatment used for severe anaemia, which is relatively common in CKD cats.

 


Symptoms of Hyperphosphataemia                                                                  Back to Page Index


There are a number of different symptoms of high phosphorus levels. You may not see all of these symptoms, and some of them may have other causes as outlined under each symptom. However, if you see any of the symptoms described below, please ensure that your cat's phosphorus level is checked (via bloodtests) and is no higher than 6 mg/dl (US) or 1.9 mmol/L (international).

 

Loss of Appetite


High phosphorus levels can make a cat feel bad and lead to a loss of appetite, particularly if secondary hyperparathyroidism develops.

 

Other causes include levels of toxins in the blood which may cause excess stomach acid, anaemia, crashing, metabolic acidosis, mouth ulcers, fluid build-up, the use of antibiotics, constipation or the use of medication for hyperthyroidism. Dental problems may also cause loss of appetite.

 

Cats who do not eat are at risk of developing a potentially life-threatening condition known as hepatic lipidosis; Mar Vista Vet has more information about this. Therefore, it is important to try to find the cause and treat it as quickly as possible.

 

Itching


Itching is fairly common in cats with high phosphorus levels, particularly if the high phosphorus levels go untreated, resulting in secondary hyperparathyroidism.

 

Itching may also be caused by general levels of toxins in the blood. Alternatively itching may indicate a vitamin B deficiency or be a sign of an essential fatty acids deficiency. Itching on the face in particular may be a side effect of the medication for hyperthyroidism. Occasionally itching can be a sign of liver problems; if this is the case, your cat's bloodwork should show elevated liver values.

 

Lack of Co-ordination in the Limbs/Back Leg Weakness


This can be due to high phosphorus levels interfering with the nerve messages that control the limbs, a condition known as neuropathy. Some of the symptoms include "forgetting" where the hind legs are (getting up and leaving without them, for instance, or leaving them in the air after licking them), or stumbling and feet crossing over when walking.

 

Plantigrade Posture


You may also see a plantigrade posture (as demonstrated by Ollie to the left), where the cat walks on his/her hocks instead of his/her feet: this is most common in diabetic cats, but may sometimes be seen in cats with high phosphorus levels, or with neurological problems from other causes. Ollie did this because of low potassium levels. .

 

Long Beach Animal Hospital has a photograph of a cat with diabetic neuropathy doing this (click on Symptoms). Newman Veterinary has a good before and after photo of a diabetic cat with this problem, scroll down a little to Other Common Consequences, then click on Plantigrade Stance (in red font).

 

University of Chicago Jack Miller Center for Peripheral Neuropathy is a human site which discusses uraemic neuropathy (neuropathy caused by CKD).

 

Teeth Grinding


This may be a sign of "rubber jaw", caused by a condition related to the CKD called secondary hyperparathyroidism.

 

Other more common causes of teeth grinding include excess stomach acid, dental problems and dehydration.

 

Animal Dental Center of Milwaukee and Oshkosh discusses the various courses of teeth grinding in cats.

Youtube has a video of a cat grinding his/her teeth.

 

Knuckling


"Knuckling" may also be seen, where the cat walks on the top of the foot with the toes tucked underneath, appearing almost to be dragging the toes behind. I haven't been able to find a photo of a cat with high phosphorus levels doing this, but there is a video on youtube of a dog with a similar problem (but with a different cause).

 

Weakness


Weakness and muscle wasting may be seen, especially in the back legs. This can be caused by high phosphorus levels leading to secondary hyperparathyroidism

 

Weakness in the back legs is often caused by low potassium levels or occasionally by low magnesium or low calcium levels; while muscle wasting may be caused by metabolic acidosis. General weakness may be caused by anaemia. If your cat no longer jumps, this may be thought to be weakness when in fact it is an unwillingness to jump because of blindness caused by hypertension. An inability to jump or climb may also be caused by arthritis.

 

If your cat suddenly cannot walk properly on one leg, particularly a back leg, and the leg feels cold to the touch, this may indicate a heart-related problem known as a saddle thrombus. This is a medical emergency, and you need to contact your vet as soon as possible.

 

Weight Loss


This may be caused by high phosphorus levels.

 

Other causes include proteinuria or  metabolic acidosis. Weight loss may also be a symptom of other diseases such as diabetes or hyperthyroidism. Other possible causes include IBD (Inflammatory Bowel Disease) or cancer.

 

Purina has a Body Condition diagram showing how to gauge your cat's physical condition.

 

Nausea


High phosphorus levels and the resulting secondary hyperparathyroidism may cause nausea, which may be manifested as a lack of appetite.

 

Alternatively, vomiting clear foam is a classic symptom for a CKD cat, and is usually caused by high toxin levels, particularly excess stomach acid. Occasionally vomiting is caused by constipation, particularly if your cat vomits before, during or immediately after using the litter tray. Anaemia or metabolic acidosis may also cause nausea. 

 

Twitching, Trembling or Shaking


Twitching may be caused by high phosphorus levels.

 

Other causes of twitching include high or low potassium levels, high blood pressure, calcium imbalances (especially head twitching), hyperthyroidism or vitamin B deficiency. Twitching may also be caused by high toxin levels. If your cat only twitches while you are giving fluids, it is probably caused by either the type of fluid used or by giving cold (room temperature) fluids.

 

Pharaoh's Shakes is a video showing a CKD cat twitching.

Pet MD mentions that twitching may be caused by kidney disease.

 


Measuring Phosphorus and Target Levels                                                     Back to Page Index


 

Your vet can determine if your cat's phosphorus levels are too high by checking your cat's bloodwork for phos, P or Pi (these are all abbreviations for phosphorus). Most vets do this routinely, but if yours doesn't, ask for it to be done.

 

Ranges for Phosphorus Levels


A typical laboratory range for phosphorus levels in cats is 2.5 - 7.5 mg/dl (USA) or 0.81 - 1.61 mmol/L (international). Unfortunately many vets see that a CKD cat's phosphorus level falls within the normal range for their laboratory and are satisfied, forgetting that the higher part of the range only applies to healthy growing kittens. Even a healthy older cat should have phosphorus levels in the bottom half of most laboratory ranges. In The role of phosphorus in feline chronic renal disease (2010) CVC in San Diego Proceedings,  Dr D Chew states "Levels of serum phosphate are often higher in young growing animals than in adults...The normal range for many laboratories unfortunately includes that of adult and growing animals, making it difficult or impossible to detect early rises in serum phosphorus above normal. Serum phosphate concentration is less than 5.5 mg/dl in most healthy adults."

 

The University of Texas Health Science Center provided the following ranges for healthy cats of differing ages a few years ago:

 

Age Phosphorus Range in mg/dl (US) Phosphorus Range in mmol/L (International)
10 days - 2 years 4.5 - 6.7 1.45 - 2.16
2 years - 12 years

4.5 - 5.5

1.45 - 1.80
Over 12 years

2.7 - 4.5

0.90 - 1.45

 

If a healthy adult cat needs phosphorus to be in the lower part of most laboratories' reference ranges, it is even more important for a CKD cat.  In Chronic kidney disease (CKD) in dogs and cats - staging and management strategies (2015) A Presentation to the Virginia Veterinary Medical Association 2015 Virginia Veterinary Conference, Dr D Chew states "Control of total body phosphate burden is an overarching goal in the management of CKD. Increasing phosphate retention during CKD occurs even as serum phosphorus values remain within the reference range."

 

Phosphorus control is also important in CKD because if the cat's phosphorus multiplied by calcium is higher than 70 (US) or 5 (international), the cat is at risk of tissue calcification (see secondary hyperparathyroidism).

 

So if your vet tells you that your cat's phosphorus level is fine, don't just take that at face value, ask for the exact level. Chances are it is too high for a CKD cat.

 

Goal for Phosphorus Level in CKD Cats


You should take action if your cat's blood phosphorus level is above 6 mg/dl (US) or 1.9 mmol/L (international). I used to recommend (as did many CKD specialists) that you try to get the level down to 4.0 mg/dl (US) or 1.3 mmol/L (international). In Chronic kidney disease (CKD) in dogs and cats - staging and management strategies (2015) A Presentation to the Virginia Veterinary Medical Association 2015 Virginia Veterinary Conference, Dr D Chew gives a slightly less ambitious target: "A targeted serum phosphate of less than 4.5 mg/dl [1.45 mmol/L] is recommended for CKD patients to ensure less phosphate burden in the bodies of these dogs and cats."

 

There is nothing wrong with these goals per se, so if you can meet them, that is fine; but as the CKD progresses, achieving them can become more challenging. The International Renal Interest Society (2013) therefore recommends a staged approach to phosphorus control, as shown in the table below.

 

Don't forget, these stages apply to stable cats; if your cat is dehydrated, for example, the bloodwork will not be accurate.

 

These values apply to adult cats. If your cat is very young (6-12 months old), then a higher phosphorus target is appropriate, because kittens need phosphorus for their bones to grow properly. Check out the table above and discuss with your vet what would be an acceptable level.

 

Phosphorus Goal: US Values


 

Stage of CKD Creatinine Level in mg/dl Phosphorus Target
IRIS Stage 2

Between 1.6 and 2.8

2.7 - 4.59 mg/dl
IRIS Stage 3

Between 2.9 and 5.0

Below 5.0 mg/dl
IRIS Stage 4

Over 5.0

Below 6.0 mg/dl

 

Phosphorus Goal: International Values


 

Stage of CKD Creatinine Level in mmol/L Phosphorus Target
IRIS Stage 2

Between 140 and 249

0.9 - 1.49 mmol/L
IRIS Stage 3

Between 250 and 439

Below 1.60 mmol/L
IRIS Stage 4

Over 440

Below 1.90 mmol/L

 

Veterinary References for Phosphorus Goals


If your vet refuses to accept that a phosphorus level above 6 mg/dl (US) or 1.9 mmol/L (international) is too high for your cat, print out and show him/her some of these references:

 

Proteinuria and renal disease: a round table discussion (2005) - on page 14 Professor J Elliott of the Royal Veterinary College states that "the goal should be to keep the serum phosphorus concentration at the lower end of the reference range."

Phosphatemia management in the treatment of chronic renal disease: a round table discussion (2006) states "practitioners need to realise that a phosphate in the normal range could still be abnormal in renal patients." Page 6 provides detailed recommendations on phosphorus control.

Dr David Polzin (2001) of the College of Veterinary Medicine of St Paul's in Minnesota states that phosphorus binders should be begun "when serum phosphorus concentration exceeds 6.0 mg/dl."

 

Monitoring Phosphorus Levels


If your cat is eating a prescription kidney diet, it may take up to a month for you to see a difference in your cat's blood phosphorus levels.

 

If you are using phosphorus binders, you should start to see a difference in blood phosphorus levels around 7-10 days after starting binders.

 

The International Renal Interest Society (2013) recommends checking phosphorus and calcium levels every 4-6 weeks to start with, reducing to every 12 weeks once your cat is stable. Dr Chew makes similar recommendations. Since in my experience most people will be visiting the vet fairly frequently in the first month after diagnosis, I suggest checking your cat's blood phosphorus level every two weeks until the level is acceptable, then checking it every 2-3 months to see if any adjustments to your treatment plan are necessary. This is worth doing because Dr Chew mentions that "Serum phosphorus concentration may increase in CKD cats that increase their food intake following other supportive CKD treatments."

 


Summary of Phosphorus Goals


 

USA


  • Take action as soon as your cat's phosphorus level goes above 6 mg/dl.

  • If your cat's level is between 4.5 and 6 mg/dl, but multiplying it by your cat's calcium level gives you a reading of 60-70 or over, take action now.

  • Your aim is to reduce your cat's phosphorus to a level of below 4.6 mg/dl. You may not be able to get it this low if your cat is in IRIS Stage 3 or 4, but aim never to let it go above 6 mg/dl.

  • You can read below about ways to reduce the level. Check phosphorus levels every month to see if any adjustments to your treatment plan are necessary.

  • If your cat is very young (6-12 months old), then a higher phosphorus level is appropriate, because kittens need phosphorus for their bones to grow properly. Check out the table above and discuss with your vet what would be an acceptable level.

  • Low phosphorus levels (below 4 mg/dl) are extremely rare in CKD cats, but are generally not considered to be a problem as long as they do not fall below 2.5 mg/dl. I would try not to go below 3 mg/dl. See Diagnosis for more information.

Rest of the World


  • Take action as soon as phosphorus goes above 1.9 mmol/L.

  • If your cat's level is between 1.45 and 1.9 mmol/L, but multiplying it by your cat's calcium level gives you a reading of 5 or over, take action now.

  • Your aim is to reduce it to a level of 1.5 mmol/L. You may not be able to get it this low if your cat is in IRIS Stage 3 or 4, but aim never to let it go above 1.9 mmol/L.

  • You can read below about ways to reduce the level. Check phosphorus levels every month to see if any adjustments to your treatment plan are necessary.

  • If your cat is very young (6-12 months old), then a higher phosphorus level is appropriate, because kittens need phosphorus for their bones to grow properly. Check out the table above and discuss with your vet what would be an acceptable level.

  • Low phosphorus levels (below 1.3 mmol/L) are not normally present in CKD cats, but are generally not considered to be a problem as long as they do not fall below 0.8 mmol/L. I would try not to go below 1 mmol/L. See Diagnosis for more information.

 


Ways to Control High Phosphorus Levels                                                           Back to Page Index


 

As discussed above, focusing on phosphorus control is one of the most effective ways to help your CKD cat feel better and live longer. The good news is, it is usually not too difficult or expensive to do this, though it may become harder to keep your cat's phosphorus levels as low as you would like as the CKD progresses..

 

There are two main ways to control phosphorus levels, in order of preference:

 

Controlling Phosphorus Levels with Diet


The first line of phosphorus control is to feed foods low in phosphorus. Retrospective analysis of dietary management of hyperphosphataemia in cats with CKD (2008) Elliott J Veterinary Record 18(2) pp45-47 concludes "This uncontrolled retrospective analysis of cats presenting to the Renal Research Clinics at the Royal Veterinary College demonstrates that feeding of renal clinical diets results in effective control of plasma phosphate concentration in about two thirds of cats presenting in Stage 2 and 3 CKD", with cats eating the renal diet living longer than the cats in the study who ate normal food with no phosphorus binder.

 

In Management of feline chronic renal failure (1998) Waltham Focus 8 (3) Dr Scott Brown states that ideally you want to feed a food with a phosphorus level under 0.5% on a dry matter analysis (DMA) basis (which is different to the percentage given on the can).

 

Most of the prescription kidney diets have phosphorus levels below 0.5%, which is one of the main reasons why these foods were developed (though they also have other attributes, see Which Foods to Feed).

 

If your cat won't eat the prescription foods (see Which Foods to Feed for tips on this), you still need your cat to eat, so you will need to look at feeding commercial foods with a low phosphorus content. In the USA, the minimum level of phosphorus in a non-prescription adult maintenance food is 0.5%, so you are not going to find a complete commercial food with phosphorus below this level; however, most non-prescription foods contain far more phosphorus than this.

 

Many people seem to think that a food below 1% phosphorus on a dry matter analysis (DMA) basis is low phosphorus but that is not the case; you want to get as close to prescription food levels as possible. If you check the food data tables in the Diet and Nutrition section, you can try to find a food which your cat will eat with the lowest possible level of phosphorus.

 

I sometimes receive excited e-mails from people who are thrilled because they have apparently found a food that does not contain any phosphorus at all, because it is not mentioned on the label. It is virtually impossible to find a cat food containing little or no phosphorus, especially if the food contains animal-based protein, as most cat foods do (phosphorus is found in meat, particularly bones). It is simply because US law does not require phosphorus to be listed on the label.

 

Although canned foods are generally a better choice for CKD cats than dry foods, since phosphorus control is so important, if I had a cat who was prepared to eat a dry prescription diet, I would feed that rather than feed a non-prescription commercial canned diet with added binders.

 

How high your cat's blood phosphorus level should be depends upon the stage of the disease. See above for more information on your phosphorus goal.

 

 Even if your cat is eating a low phosphorus diet, at some point it is quite possible that this will become insufficient to control your cat's phosphorus levels. In The role of phosphorus in feline chronic renal disease (2010) CVC in San Diego Proceedings,  Dr D Chew states "Diet alone is not successful in adequate phosphorus control as chronic renal disease becomes more advanced. In these instances serum phosphorus concentration increases above the normal range or stays in the upper half of the normal range." In this situation, you will probably have to consider the addition of phosphorus binders.

 

If your cat's phosphorus level is normal, it is still wise to feed a food as low in phosphorus as you can, because the less phosphorus your cat eats, the less work there is for the kidneys to do trying to process it. Also, phosphorus levels tend to rise as the CKD progresses, and usually it is easier to keep them low rather than have to work on reducing them.

 

See above for information on how to monitor your cat's phosphorus levels.

 

Controlling Phosphorus Levels with Phosphorus Binders


If your cat will not eat a prescription diet, or if his/her phosphorus levels are still too high (over 6 mg/dl (US) or 1.9 mmol/L (international)), despite feeding such a diet for a month, products called phosphorus binders are used. These are simply added to the cat's food: they bind with some of the phosphorus in the food in the intestine, thus preventing it from being absorbed and thereby reducing levels of phosphorus in the cat's body.

 

Survey of dietary and medication practices of owners of cats with chronic kidney disease (2014) Markovich JE, Freeman LM, Labato MA and Heize CR Journal of Feline Medicine and Surgery 22 found that only 22% of the 1080 cats in the survey were receiving phosphorus binders. Not all of the cats may have required binders at the time of the survey but this is still a very low percentage considering the consequences of not controlling phosphorus levels in CKD.

 

There is a separate page all about phosphorus binders.

 

 

 

Back to Page Index

This page last updated: 04 May 2015

Links on this page last checked: 04 May 2015

   

 

*****

 

TREATING YOUR CAT WITHOUT VETERINARY ADVICE CAN BE EXTREMELY DANGEROUS.

 

I have tried very hard to ensure that the information provided in this website is accurate, but I am NOT a vet, just an ordinary person who has lived through CKD with three cats. This website is for educational purposes only, and is not intended to be used to diagnose or treat any cat. Before trying any of the treatments described herein, you MUST consult a qualified veterinarian and obtain professional advice on the correct regimen for your cat and his or her particular requirements; and you should only use any treatments described here with the full knowledge and approval of your vet. No responsibility can be accepted.

 

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