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What is PKU, or Phenylketonuria? Call the Midwife diagnosis explained

In episode five, the midwives and Dr Turner come across a child with a rare genetic disorder.

Call The Midwife - Ep5
Published: Sunday, 16th May 2021 at 8:24 pm

Over the years, Call the Midwife has taught us many lessons about medicine and childbirth – but in season 10 episode five, we're talking about a literal lesson as Shelagh Turner (Laura Main) stands in front of her blackboard quizzing her pupil midwives about "post-natal testing for mother & baby".


One of the things mentioned is a urine test for PKU, or phenylketonuria, which Shelagh describes as "exceptionally uncommon". Sadly, it's not uncommon enough and, later in the same episode, Sister Julienne (Jenny Agutter) comes across a child suffering from the condition.

So what do we know about this disorder? From symptoms to diagnosis, here are the details.

What is PKU, short for Phenylketonuria?

Phenylketonuria (PKU) is an inherited disorder, which can be very serious if left untreated – and which cannot be cured (at least, not yet). However, it can be very effectively treated and controlled.

After young Elaine is diagnosed, Dr Turner (Stephen McGann) tells her devastated parents Vera (Paula Lane) and George (Daniel Easton): "Elaine was born unable to rid her body of a chemical in her diet called phenylalanine. This chemical built up in her bloodstream and has had a harmful effect on her brain."

To explain this a little further: we consume protein in many of the things we eat, such as meat and fish. During digestion, our bodies break the protein in the foods down into amino acids, which are the "building blocks" of protein. We then use these amino acids to make our own proteins; any amino acids which are not needed are broken down further, and expelled from the body.

But people with PKU do not have the ability to break down the amino acid "phenylalanine". So when they eat and digest protein, that particular amino acid builds up in the blood and the brain, which can lead to brain damage.

How is PKU diagnosed?

Season 10 of Call the Midwife is set in 1966, when Shelagh explains to her pupils why the urine test is so important: "PKU is a disease that's best caught early. And the test is a recent discovery. Sometimes we test because we can test. No information, however we gather it, can ever be said to be wasted."

Call The Midwife - Ep5

As we learn from Shelagh's lesson, the urine test cannot be carried out until the baby is about three weeks old. Sister Julienne demonstrates how it's done later in the episode, when she presses the test strip into Elaine's nappy; it changes colour to green, indicating a positive result for PKU.

Later in the episode, Sister Julienne shows us the other way of testing for PKU in a newborn baby, which can be carried out sooner after the birth than the urine test. In order to find out whether Vera Sands' newborn has also inherited the disorder, she instead collects a blood sample to send in for tests.

The latter approach is how today's newborns are tested for PKU in the UK. As the NHS website says: "At around five days old, babies are offered newborn blood spot screening to test for PKU and many other conditions. This involves pricking your baby's heel to collect drops of blood to test. If PKU is confirmed, treatment will be given straight away to reduce the risk of serious complications."

What are the symptoms of PKU?

Symptoms of untreated or undiagnosed PKU can include eczema; repeated vomiting; jerking movements in arms and legs; tremors; epilepsy; a musty smell to the breath, skin and urine; microcephaly (a small head); fairer-than-usual skin, hair and eyes (as phenylalanine is involved in the body's production of melanin); behavioural difficulties; and learning difficulties and developmental delays, caused by damage to the brain and nervous system.

These symptoms will start to appear in babies a few months after birth, and – if the disorder is left untreated – the effects can be irreversible.

However, if treatment is started soon after birth, PKU does not usually cause any symptoms at all. The NHS estimates that approximately one in every 10,000 babies born in the UK has the condition, but most are able to live healthy and symptom-free lives.

How is PKU treated?

The main treatment for PKU is a special, low-protein diet.

High-protein foods such as meat, eggs and dairy products must be completely avoided, while the consumption of other foods containing protein (such as potatoes) must be carefully controlled. The artificial sweetener aspartame should also not be eaten.

People with PKU will also be given an amino acid supplement to take, and their phenylalanine levels will be monitored with regular blood tests. The NHS says: "As long as a person with PKU sticks to a low-protein diet throughout childhood, and their phenylalanine levels stay within certain limits, they'll remain well and their natural intelligence will be unaffected."

Some people with PKU do eat a more "normal" diet once they reach adulthood. However, the advice is to remain on a low-protein diet for life. This is especially important for pregnant women with PKU, to avoid miscarriage or damage to the foetus.

Is PKU inherited?

Yes, The genetic mutation responsible for PKU is passed on by the child's parents; they are usually "carriers", and do not have the condition themselves.

Paula Lane plays Vera Sands in Call the Midwife
Paula Lane plays Vera Sands in Call the Midwife (BBC)

The mutation is passed on by "autosomal recessive inheritance", so a baby needs to receive two copies of the mutated gene to develop PKU: one from the father, and one from the mother. A baby who only receives one affected gene will also become a carrier, but will not develop the disorder.

In Call the Midwife, Elaine and George Sands are both carriers – not that they had any idea before Elaine's diagnosis. That means their offspring will have a one-in-four chance of inheriting PKU (as Elaine did); a one-in-two chance of being a carrier of PKU; and a one-in-four chance of receiving two normal genes.

When was PKU discovered?

As Shelagh indicates, in 1966 the testing programme for PKU was a relatively recent development.

Phenylketonuria was identified by the Norwegian physician Ivar Asbjørn Følling in 1934, when he found excess phenylpyruvic acid in the urine of two siblings who'd developed intellectual disabilities.

It then took some time for the condition to be fully understood, and for proper treatment to be developed – in the form of a low-protein diet.

But around 1961 there was a crucial development: American scientist Robert Guthrie developed a test that could be used to check infants for PKU after birth, and screening programmes were quickly introduced around the world.

At first, testing wasn't very joined-up (which is why Elaine slipped through the net), but by 1968 England had an established national screening programme.


Call the Midwife continues on Sundays at 8pm on BBC One. While you're waiting, take a look at the rest of our Drama coverage, or find out what else is on with our TV Guide.


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