Hypertonic saline infusion test (with copeptin measurement)

What is a hypertonic saline infusion test?

Your doctor has requested that you have a hypertonic saline infusion test (HSIT) with copeptin measurement. This is to investigate whether you may have arginine vasopressin (AVP) deficiency.

This condition is also known as central diabetes insipidus. It can lead to an inability to concentrate urine, leading to large urine volumes, thirst and increased fluid intake.

The hypertonic saline is a salt solution that aims to raise your blood sodium levels. This should stimulate the hypothalamus and pituitary to release the hormone AVP.

AVP acts on the kidneys to concentrate urine and reduce urine output. Because copeptin is released at the same time and is more easily measured; it is used as an indicator for the AVP concentration in your blood.

How you need to prepare for the test

In preparation for the test, please:

• Fast (no food) from midnight before the test.
• Only drink plain water during the fasting period.
• Do not drink fluids (water, tea, coffee, etc.) for at least two hours before the test starts. This start time will be indicated in your appointment letter.
• Avoid nicotine and alcohol for 24 hours prior to the test.
• Avoid strenuous exercise for 24 hours prior to the test.
• If you are taking a medication called Desmopressin (DDAVP), you should usually stop taking it 24 hours prior to your visit (or omit your 20:00hrs/8pm dose the night before). This can be discussed with your medical team.
• Bring all of your regular medications with you.
• Let us know if there is any possibility that you may be pregnant.

What happens on the day of the test?

Please come to the Endocrine Investigation Unit, Wolfson Diabetes & Endocrine Clinic (Clinic 32), Addenbrooke's Treatment Centre at Addenbrooke's Hospital and check in at reception. The test should last no more than four hours. You may wish to bring something to read or occupy yourself with (a quiet activity).

Upon arrival, we will measure your weight and height. We will also need to collect a small sample of urine.

Two small plastic cannulae (thin tubes) are placed in a blood vessel of each arm:

• one is used for the hypertonic saline infusion
• the other is used for regular blood sampling during the test

Your blood pressure and heart rate will also be checked regularly, and we will ask you before and after the test about any symptoms you may be experiencing (for example thirst, nausea, headache).

Once the sodium level in your blood has risen above a certain level, we will stop the infusion and collect the blood for the copeptin test. Then to reduce your sodium to its initial level, we will ask you to drink a specified amount of water within 60 minutes. The amount is related to your size and is calculated at 30ml/kg body weight, for example 2.1L with a body weight of 70kg) within 60 minutes. In addition, you will receive an infusion of glucose.

Afterwards, your sodium level is checked for the last time. Then the plastic catheters are removed and you are allowed to eat something.

We aim that your sodium will have returned to normal, and that you are feeling well, before you leave the endocrine unit to go home.

Will I experience any side effects?

Most patients will feel thirsty during the test. The next most common symptoms are mild headache or malaise, each of which may affect around half of people having the test. Mild nausea is a less common experience.

When will I get my results?

The results can take some time to process and interpret. They will be discussed with you at a later date by your medical team.

MyChart

We would encourage you to sign up for MyChart. This is the electronic patient portal at Cambridge University Hospitals that enables patients to securely access parts of their health record held within the hospital’s electronic patient record system (Epic). It is available via your home computer or mobile device.

More information is available on our website: MyChart.

Contacts/further information

Please contact the Endocrine Investigation Unit and leave a message on 01223 217848 (voicemail) from 09:00 (9am) to 16:00 (4pm), Monday to Friday.

References/sources of evidence

A Copeptin-Based Approach in the Diagnosis of Diabetes Insipidus. Fenske et al. N Engl J Med 2018;379:428-39.