O'Sullivan Test
The O'Sullivan test is a glucose tolerance test that checks whether the body processes sugars properly and is used to diagnose gestational diabetes (diabetes during pregnancy).
General Description
The O'Sullivan test is a routine screening performed on pregnant women to identify the risk of developing gestational diabetes. Due to changes that occur in the body during pregnancy, this glucose tolerance test is very important, as placental lactogen prevents insulin from acting properly, leading to increased blood sugar levels.
This test is used as a screening tool, meaning it is performed on all pregnant women who do not have a prior diabetes diagnosis. If necessary, treatment or lifestyle changes are implemented to reduce the risks associated with this condition during pregnancy, which may include preterm birth, macrosomia (birth of a baby weighing more than four kilograms), fetal brain development abnormalities, respiratory difficulties, or complications during delivery.
When is it indicated?
In Spain, the O'Sullivan test is performed on all non-diabetic pregnant women between the 24th and 28th weeks of gestation.
In some cases, a glucose intolerance test is required during the first trimester, for example, in women with a history of gestational diabetes, obesity, or prediabetes. It is also indicated when prenatal tests suggest that the fetus has an unusually large size and weight.
How is it performed?
The O'Sullivan test is conducted through an oral glucose load, meaning a specific amount of glucose is ingested to observe how the body reacts. This test, commonly known as the glucose curve due to the shape of the results graph, is carried out in several phases:
- A baseline blood test is performed to measure fasting blood sugar levels.
- The patient drinks a solution containing 50 grams of glucose and then rests.
- After 60 minutes, a second blood test is conducted.
Only if the results of this initial test are abnormal is a second, longer test required four weeks later. The process begins similarly to the first test but extends further:
- After 120 minutes, a third blood test is performed.
- After 180 minutes, the final blood test is taken.
The blood extraction process follows standard procedures:
- The patient sits or lies down on an examination table with the arm extended.
- The area is disinfected, a tourniquet is placed above the elbow, and the patient clenches their fist to facilitate the draw.
- In most cases, the needle is inserted into a vein inside the elbow. At the same time, the fist is relaxed, and the tourniquet is released.
- The sample is collected in a gray-topped test tube. These tubes contain an anticoagulant (sodium fluoride) that halts glycolysis, the process by which carbohydrates are metabolized. This ensures that the blood remains in its natural state for up to 72 hours.
- In the laboratory, the samples are centrifuged to separate the serum, and a glucose reagent is added to each one before allowing them to rest.
- The mixture is analyzed using an automated electronic device, providing information on the blood sugar levels at each stage of the test.
Risks
The primary risk associated with the O'Sullivan test is the possibility of dizziness, particularly during the longer test, as the patient must remain fasting while multiple blood samples are drawn.
Additionally, some women may vomit after consuming the glucose solution, requiring the test to be rescheduled.
Occasionally, bruising may occur at the puncture site.
What to expect from the O'Sullivan test
On the day of the glucose curve test, it is advisable to wear comfortable clothing, as the arm will need to be exposed multiple times for sample collection. The patient will need to remain at the testing center for between 90 and 200 minutes. During this time, eating, smoking, and walking are not permitted.
Once the first blood sample is taken, the glucose solution must be consumed within five minutes. Due to its texture and taste, it may be unpleasant and even cause nausea. The best approach is to drink it slowly to avoid vomiting but without taking too long.
Women who feel dizzy during the test can place their head between their knees to alleviate the sensation. If the dizziness persists, they will be asked to lie down on an examination table with their legs slightly elevated for the remainder of the test.
It is recommended to apply pressure to the puncture site for a few minutes to prevent bruising, especially during the longer test, which requires multiple blood draws.
Results are usually available a few days after the test. The O'Sullivan test is considered positive if blood glucose levels one hour after ingestion of the solution are equal to or greater than 140 mg/dL.
If the O'Sullivan test is positive, a glucose tolerance test is performed to confirm the diagnosis. However, specialists consider additional factors before making a final diagnosis, and test results must always be interpreted by a physician.
Medical specialties that request the O'Sullivan test
The O'Sullivan test is conducted by clinical analysis specialists at the request of endocrinologists or gynecologists.
How to prepare
To undergo the O'Sullivan test, the patient must fast for eight hours beforehand. Smoking and physical exercise should be avoided before and during the test.
