Monitoring your baby’s Phe levels

Article 16 Monitoring PHE levels

If blood Phe levels are too high over a period of time, it may affect a baby’s developing brain, and this may lead to later problems with learning, mood and behaviour. If Phe levels are too low for a prolonged period of time, it may slow growth.

It is normal for Phe levels to change over the course of the day and should not be a source of worry. A number of factors can affect levels. It is helpful to know what your baby’s Phe target range is and if levels are higher or lower than this, it is useful to try to figure out why. Your metabolic healthcare professional team in particular the dietitian, will work with you on determining the cause of high or low levels, however over time, it will become easier for you to work this out. In time, you will also develop your own routine for testing monitoring and maintaining your baby’s Phe levels, but initially will be given clear instructions.

Factors affecting the Phe levels of your baby

Time of Day

Blood Phe levels are usually highest in the morning. This is normal and happens to all children and causes no harm. It happens because the body starts to break down some of its protein stores (protein contains Phe) overnight and body tissue contains Phe. However, in very young babies who feed regularly at night, this is not the case, because the body has an energy source from the overnight feeds and does not need to break down body stores. When testing Phe levels, it is often recommended to test levels at the same time of day to reduce variation. Consistency in the time of taking blood tests, and making sure they are taken before rather than after a feed is important.


If your baby is sick, has an infection or has lost their appetite, his or her body will break down its own protein stores to use as energy. This occurs in all children but in those with PKU, this releases Phe into the blood, causing levels to rise. Levels should return to normal when your baby is well again. If your child is sick for more than a day or so, or they get sick frequently it is helpful to discuss this with your metabolic healthcare professional team, as they will be able to give you some ideas to help keep Phe levels from rising too much.

PKU Formula

Using PKU formula (which is Phe-free) will help to lower your baby’s blood Phe levels. This is because the body needs a regular supply of energy and protein or it has to break down body tissue. Taking PKU formula frequently (3-4 times a day, or more often if your baby feeds every few hours) helps prevent this from happening. This is most important in young babies with high energy needs who are growing quickly.

Weight Loss

If your baby becomes ill, does not get enough energy and does not grow well or loses weight as a result, this may also cause blood Phe levels to rise. When weight loss occurs the body is breaking down tissue. The Phe in the body tissue makes blood levels rise, especially if weight loss occurs too quickly. All children need to grow steadily, so even a long period of time with poor growth may have an effect on Phe levels.

Phe from Food and Drink

As natural protein from breast milk, infant formula or foods contains Phe, eating or drinking more natural protein than the body needs also increases blood Phe. This is why, as your baby gets older and you begin to introduce foods into the diet, breast milk or infant formula amounts will need to be exchanged for food protein. Particularly in the early years while you are getting used to this, it is useful to measure food to avoid eating more natural protein and Phe than prescribed.

Phe levels may increase when:

  • Protein intake (and therefore Phe intake) from breast milk, standard formula or food is too high
  • Intake of PKU formula is too low which causes the body to break down its own stores for nutrients, releasing Phe into the bloodstream
  • Your baby’s rate of growth slows, meaning less Phe is being used to build new proteins
  • Your baby is unwell, which may make it difficult to drink PKU formula, and cause higher Phe levels from the breakdown of the body’s own muscle tissue
  • Phe containing breast milk or regular formula or food and PKU formula are not spread well over the day.

Phe levels may decrease when:

  • Intake of Phe or protein from breast milk, standard formula or food is not enough for your child’s needs at that time.
  • Rapid growth in a child causes levels to fall as Phe is rapidly used to build new muscle tissue
  • In these cases, your dietitian will recommend how much more protein you need to give your baby.

How to take a Blood Sample

Phe levels are usually measured using a small blood sample taken from the heel of babies and toddlers and from the fingertip as children grow older. Parents or caregivers are taught how to collect samples from babies and young children at their clinic visits. The procedure is easy to manage once you have had a little practice. It is important to communicate to your child through your actions and words that getting blood samples is vital to their health. It is not always easy to convince a child to check their blood levels and your metabolic healthcare professional team will help you with that. Helping your child learn the importance of blood samples will help them to maintain good control of their PKU over time. At this time blood levels cannot be measured on a home device, but need to be sent to a laboratory that has the specialised equipment to measure them. This means that the result will reflect what was happening with blood Phe levels from a few days before you get the result.

Blood Sampling Tips

  • Involve your child as soon as you can
  • Blood samples are a fact of life for children with PKU. The positive patterns you help establish when your child is young will provide a solid foundation for how he or she manages blood samples throughout life.
  • Increase circulation to the hands or feet
  • Make sure the hand or foot is warm before taking the sample. Soak in warm water, gently rub the area, or sit in a warm room before taking the blood sample.
  • Try different spots on the finger
  • The side of the finger may be less painful because there are fewer nerve endings. Rotate the site you do the test and do not use any area of broken skin/bruising.
  • You can use a topical analgesic to reduce pain and anxiety with finger sticks, however it is likely that this will only be needed short term as blood tests are taken frequently throughout childhood.
    Ask your metabolic healthcare professional team for more information.

How frequently should I test?

During infancy your baby is growing rapidly. As a result, monitoring blood Phe levels frequently is important as they may fluctuate. During this time your clinic policy may require taking samples several times per week initially. Once your baby has settled into their diet and growth has slowed, the frequency of blood samples may be reduced. The frequency of blood tests may also change as your baby grows into a child and adolescent. Your metabolic healthcare professional team will determine what is best. Extra blood samples (as directed by your clinic) may be needed during or after an illness to figure out whether Phe levels are too high or too low.

Please Note: The dietary management for PKU varies for each person so all information presented here is for guidance only. Your own dietitian and/or doctor will advise you on all aspects relating to management of PKU for you and your family.

My PKU Binder [2011]. National PKU Alliance (USA). Chapter 3: Monitoring Blood Phenylalanine levels.
PKU Handbook [2005]. Australasian Society for Inborn Errors of Metabolism (ASIEM) (Australia and New Zealand). Chapter 5: Collecting Blood Samples.

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