The Theory Behind
The inner surface of the bladder is covered by a glycosaminoglycan layer; this is called GAG-layer. This protects the tissues of the bladder from all the irritative constituents of the urine. In the case of IC/BPS (and certain other conditions), this GAG-layer is deficient. It does not protect the pain nerve endings situated behind the bladder mucosa from the irritative salts and compounds, which results in the typical syndromes of severe pain and the frequent desire of voiding.
In the past, several methods were developed to estimate the condition of this GAG-layer. During these tests, certain solutions were instilled into the bladder. However, these methods were invasive and (in the case of certain tests) painful, too, and their accuracy was inadequate. Current guidelines list none of them among the recommended diagnostic methods.
The GAG-layer integrity test is based on the fact that for examining the effect of irritative substances on the bladder, no solution need be instilled, since the irritative constituents (e.g. potassium) are present in the urine itself. In conclusion, the more concentrated the urine is, the higher the irritative effect is – and the worse the symptoms are.
Normally, in the case of a healthy GAG-layer, the more someone drinks, the more urine is being produced, obviously. However, the average of urine volume of the voiding occasions (the urine portions) is roughly the same. If the GAG-layer is deficient, the situation is different. The large fluid intake dilutes the urine, which results in a lower concentration of the irritative constituents; their irritative effect is more negligible. Thus, the symptoms are less severe. Therefore, patients with a deficient GAG-layer (typically, IC/BPS patients) can hold their urine back longer if it is less concentrated, which leads to bigger urine portions. In conclusion, the more expressed the GAG-layer deficiency is, the bigger the difference between the urine portions produced in case of concentrated and dilute urine is.
By applying the GAG-layer Integrity Test, this difference can be quantified. This means in the case of GAG-layer test, that we can define the difference between the mean voided volumes at concentrated and diluted urine objectively, by numbers, expressed in percents.
The test has several other advantages, as well: it is not invasive, painless, and can be performed by the patients on their own.
How Is the GAG-layer Integrity Test Performed?
It is a new method of diagnosing IC/BPS, and has been successfully applied – both for diagnostic and follow-up purposes – for years.
For estimating the status of the GAG-layer, the urine portions have to be measured in case of concentrated and dilute urine, as well. To do so, on the first day of the test (Day 1), the patients are to have the least fluid intake possible (approximately 1.5 liters in the summer, 1 liter in the winter). On the second day of the test (Day 2), they have the most fluid intake possible (3.5 liters in the summer, 3 liters in the winter). It is crucial to hold back the urine as long as possible. Each portion is to be measured during the daytime – the difference between the average volume of portions of Day 1 and Day 2 refers to the integrity of the GAG-layer.
(Many other voiding diaries are taking nighttime voiding into account, too. Our experience shows that it is not necessary. Nighttime voiding is affected by several factors not directly related to any condition of the bladder. Therefore, taking these portions into consideration would adversely influence the accuracy of the test.)
Practical Instructions for the Voiding Diary
To perform the test correctly, the instructions below are to be followed precisely:
Since there are many measurements to be performed, it is worth using a regular kitchen scale (and measure the mass) instead of a measuring glass (for the volume). By doing so, any glass can be used for measuring the urine. Put the empty glass on the scale, null it, and put the full glass on the scale afterward. Although you get the mass (in grams or ounces) of the urine and not the volume (in milliliter or fluid ounces), the two sorts of data are correlated to each other. In addition, since only the difference between the two days’ portions counts, the mass can be used for the calculation, too.
The result is more accurate if the difference between the fluid intake on the two days is considerable. It is worth cutting down on drinking already from the afternoon of the day before Day 1 (Day 0).
Always try to hold your urine back as long as you can (maximal bladder capacity).
Ignore the first voiding (after waking up) and the last voiding (before going to sleep) of the day. The first one in the morning is usually much bigger than the others, and the last one int the evening is much smaller, since most people void before going to sleep for “safety reasons”.
Ignore any voiding which happens for any other reason than feeling that the bladder is full. (A typical case for this is a “preventive voiding”, just before someone must leave home.)
On Day 2, try to drink at least 1.5 liters in the first 90 minutes after waking up. Then, during the day, drink another 2 liters, most preferably in a balanced manner; 200 ml in each hour, if possible.
A large amount of fluid intake dilutes the blood, too. The urine produced by the kidneys is affected by several hormones. These result in a delay of 2–2.5 hours regarding the urine portions. Therefore, ignore any portions in the first 2–2.5 hours after waking up on Day 2.
The test will be more accurate if you consume the same sorts of food during the test. By doing so, no compounds originated from the metabolism of different food types will affect the urine portions – only the amount of fluid intake will. Try to drink water, weak chamomile tea, or linden blossom tea. No other dietary prescriptions need be followed.
If you have to take any medicine during the test (e.g., antihypertensives, diuretics), do not change its dosage from the day before Day 1. Any changes in the administered medicine may affect the amount of urine, thus, the accuracy of the test, too.
S. Lovasz MD. Ph.D.