There is no one test to diagnose chronic lung disease. A doctor may first suspect it if your baby has difficulty breathing. The diagnosis is confirmed when one of the following is present:

• The baby needs extra oxygen for at least 28 days after birth.
• At 36 weeks of gestational age, the baby needs more oxygen than is present in ordinary air. Gestational age is the number of weeks and days a baby has developed since the beginning of the pregnancy, or gestation.

Babies with chronic lung disease usually have regular blood tests (including blood gases) to monitor how well their lungs are working. These tests may be done until the baby can breathe without extra oxygen.

A number of tests may be done to rule out other causes of difficulty breathing and to learn whether complications of chronic lung disease have developed.

• A baby may have an electrocardiogram and an echocardiogram to see how well the heart is working. Echocardiograms are usually repeated every 2 to 3 months until 4 to 6 months after oxygen therapy has stopped.
• A baby may have a lung function test to find out how much chronic lung disease has damaged the lungs. This test is repeated regularly as the child gets older. After results are normal, a child may no longer need lung function tests.

See an illustration of chronic lung disease.