A small amount of CK is normally present in our blood, and we can measure it with a simple blood test. When CK levels are higher than normal, it often means the muscles are under stress or have been damaged. In Duchenne and Becker muscular dystrophy, the body lacks a protein called dystrophin, which helps keep muscle cells strong and stable. You can think of the cell membrane as the outer covering of a muscle cell. It controls what goes in and out. Without enough dystrophin, the membrane becomes fragile, and CK can leak out of the muscle cells into the blood, leading to higher CK levels on a test.

CK is very sensitive for Duchenne and Becker, meaning it’s usually elevated in almost everyone with these conditions. However, it’s not a perfect test. CK levels can be influenced by many factors, including the age at which the test is done. In general, a normal CK level in a young child is quite reassuring and makes Duchenne less likely. Most children with Duchenne will have a high CK level early in childhood.

There are many reasons a CK level can be elevated. Some common causes include muscle injury or stress, inflammation in the muscles, certain medications, or even severe illness. Strenuous exercise can also raise CK levels. Additionally, there are other muscle disorders, besides Duchenne and Becker, where CK may also be elevated.

CK levels can fluctuate somewhat over time, both in the short term and long term. In people with Duchenne, CK levels are usually highest in early childhood and then gradually decrease as a person gets older. This drop tends to follow the loss of motor function. As muscles weaken, there is less healthy muscle tissue, and over time that muscle is replaced by fat and scar tissue. Because of this, there’s less CK released into the blood, which is why CK levels tend to go down with age.

CPK stands for creatine phosphokinase, and it’s essentially the same thing as CK, or creatine kinase. The terms are used interchangeably, though CPK refers a bit more specifically to the chemical reaction the enzyme helps with. CK actually exists in a few different forms, sometimes called isoforms. Different tissues in the body make different amounts of each form. For example, CK-MM is mostly found in skeletal muscle, CK-MB is found in the heart, and CK-BB is found in the brain. When we measure a total CK, that number represents the sum of all these isoforms. If any one of them is elevated, it contributes to a higher total CK. Looking at which isoform is elevated can help determine where the CK is coming from, whether it’s the muscles, the heart, or the brain.

CK levels can help point us in the direction of a possible muscular dystrophy, but other tests are needed to make a specific diagnosis. Think of CK as a screening tool. It can raise suspicion for Duchenne, Becker, or other types of muscular dystrophy, but additional testing, usually genetic, is needed to confirm what is going on. A normal CK level also does not rule out a muscle disorder completely. Some congenital myopathies or other muscle conditions can have normal CK levels. Most muscular dystrophies, however, will show an elevated CK. So it is a helpful first step in the diagnostic process, but it is just the starting point.

It would be very unusual for a child with multiple normal CK levels to develop Duchenne in the future. While CK is not very specific for the type of muscular dystrophy, it is usually very high in patients who do have muscular dystrophy. Patients who have had normal CK levels, especially on multiple occasions, are very unlikely to develop muscular dystrophy in the future.

CK is not a reliable way to determine who is a carrier of a DMD gene mutation. Some female carriers may have mildly elevated CK levels, but many carriers have normal CK levels, so a normal result does not rule out carrier status. In cases where a female with a known DMD mutation has a high CK level, it could suggest she is a manifesting carrier, and specialists may pay closer attention to potential symptoms. However, CK alone cannot determine the presence of heart or other symptoms in carriers. Identifying subtle symptoms in carriers requires a detailed evaluation by a specialist, not just a CK test.

If a person has a normal CK and a genetic test identifying a variant, it generally rules out Duchenne or Becker muscular dystrophy, especially if CK has been normal on multiple occasions. Some variants in the DMD gene may be benign, so interpreting the results usually requires guidance from a neuromuscular specialist or a genetic counselor familiar with the DMD gene.

The best approach is to ask your doctor to help interpret the results. If your primary care doctor ordered the test, start there. They may know the answer or may need to consult a specialist. You can also take the question directly to a neuromuscular specialist. CK levels can be tricky to interpret, so getting input from a specialist can be very beneficial.

The most important thing is simply to check CK levels, especially in any child with motor delays. Checking CK early and, if needed, checking it regularly can help identify muscular dystrophy sooner. A very high CK does not automatically predict what will happen in the future. High CK can have many causes, and even when it reflects muscular dystrophy, outcomes are not set in stone. Advances in care and new treatments continue to change what is possible, so the future remains uncertain. The key is early evaluation and monitoring, not assuming results dictate the long-term course.

There are several reasons a newborn screen for Duchenne might be false positive, mainly because the test uses CK rather than genetic testing. While CK is very sensitive for Duchenne, it is not 100% specific. Common causes of false positives include birth trauma, infection during the neonatal period, and other muscle disorders. CK levels are also naturally higher during the first 24 to 48 hours after birth, since the physical process of birth, even a C-section, can be demanding for newborns. The timing of the screen and whether it is repeated can help determine whether an elevated result is more or less likely to be a false positive.

Ideally, the goal of newborn screening is to detect every case of Duchenne. However, no screening test is perfect. There can be variability in lab cutoffs, which determine how high CK must be to trigger a positive screen. Some factors can elevate CK, while others can lower it. Prematurity and very low birth weight are known examples. If the CK measurement falls below the cutoff, it is possible the baby could be missed by newborn screening. This is uncommon, but it has been observed in previous pilot programs. Finally, while very rare, there is always a small risk of technical or human error.

Unfortunately, the answer here is not straightforward. For both people with Becker muscular dystrophy and DMD carriers, CK elevation at birth is more variable than in Duchenne. However, pilot studies and newborn screening programs that have been implemented so far have successfully detected both.

First and foremost, it’s important to try to remain calm. CK is a screening test and by itself is not diagnostic. The evaluation pathway can vary depending on your state. It might involve repeating the test, since CK is typically highest in the first few days of life when the newborn screen is usually done and often normalizes afterward. Alternatively, the next step could be a referral to a genetic or neuromuscular specialist. For parents or family members, it is important to be selective about the resources you use for information. For healthcare providers, it’s recommended to review your state’s screening protocol and discuss next steps with neuromuscular colleagues.

It’s important to remember that newborn screening programs are state-dependent. You can’t assume your state screens for Duchenne just because other states do. Even if your state does include Duchenne in its newborn screening, any child presenting with developmental delays should still have a CK test. CK is a valuable screening tool both in the newborn period and later in life, so it should always be considered when evaluating a child with developmental delays.