URINARY OUTPUT: CAUSES, HOW IT IS CALCULATED AND NORMAL VALUES - MEDICINE - 2024

The urine output is the amount of urine per kilogram of body weight a person produces over a period of 24 hours. It is a very important indicator of general health and when it is altered, the cause must be investigated in order to avoid serious and in many cases irreversible complications.

Diseases that modify urinary output are generally related to the kidneys, however dehydration, some metabolic diseases such as diabetes and even some tumors can cause urinary output to increase or decrease.

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Rarely do we think about how many times we urinate and how much, however, for doctors this information is very important, especially in certain clinical contexts, such as the critically ill patient or one hospitalized in the Intensive Care Unit.

Likewise, in patients with kidney diseases, collagen diseases and metabolic problems such as diabetes, it is very important to know the urinary output since it is directly related to the degree of functioning of the kidneys.

Urinary output can be modified according to the clinical context, both situations being delicate, since they can be associated with very serious complications that could lead to irreversible injuries for the patient and even compromise his life.

Causes

Since the production of urine is a natural mechanism of the body to get rid of excess fluids as well as an infinity of toxins that are eliminated by the kidneys, it can be said that urinary output is a direct consequence of kidney function.

Therefore, its modification should make us think that there is a problem in any of the phases of urine production, that is, at the pre-renal, renal or postrenal level.

In this sense, situations that decrease urinary output and those that increase it can be defined.

Causes of increased urinary output

Urinary volume increases in certain clinical conditions such as diabetes mellitus and diabetes insipidus, to name just two of the causes of increased urinary output.

The mechanisms are different for each of them, although the common manifestation is an increase in the amount of urine that is produced in 24 hours.

In the case of diabetes mellitus, the increase in the amount of glucose in the blood and therefore in the urine, generates what is known as «osmotic diuresis», that is, the sugar attracts water to the renal collecting system, making the volume urinary increase.

For its part, in diabetes insipidus, the mechanism of action is totally different. In these cases there is an inadequate secretion of a hormone that facilitates the absorption of water in the kidney in order to prevent it from being lost in excess.

When this substance, known as antidiuretic hormone (or vasopressin), is produced in insufficient amounts, urinary output increases significantly.

Causes of decreased urinary output

There are multiple causes of decreased urinary output, one of the most common being dehydration.

As there is less water in the body, the kidneys start to work in what could be called "saving mode", that is, they eliminate the least amount of water possible in order to prevent the intensity of dehydration from increasing. When this happens the urinary volume decreases dramatically.

Fortunately, it is a reversible condition and easy to treat, however when dehydration persists it can cause irreversible kidney damage, causing urinary output to remain below normal due to kidney failure.

In this sense, in addition to dehydration there are countless diseases that can generate alterations in the kidneys that over time make them stop working properly, decreasing urinary output in a sustained and in many cases irreversible way.

Among the most common causes of kidney damage are diabetes mellitus (diabetic nephropathy), high blood pressure (hypertensive nephropathy), autoimmune diseases (such as lupus nephritis), and degenerative kidney diseases (such as polycystic kidney).

In each of the previously mentioned clinical conditions there is a specific mechanism of kidney damage, although ultimately the loss of functional kidney tissue leads to a decrease in the ability of the kidneys to produce urine and consequently there is a decrease in urinary output.

In the most severe cases, total loss of kidney function can be achieved, with very low or zero urinary output, which is why it is necessary to supply kidney function with dialysis in order to keep the patient alive.

How is urinary output calculated?

There are two methods to calculate urinary output, one direct and one indirect. The former is often used in the clinical setting, especially in operating rooms and intensive care units since it is necessary to manipulate and invade the urinary tract to determine the volume of urine produced.

For its part, the indirect method is usually used at home and requires the collection of all the urine produced during 24 hours to be able to perform the corresponding calculations.

Indirect Calculation

Indirect calculation of urinary output is the most used method to have an objective idea of ​​kidney function.

Although it is somewhat cumbersome and annoying, in order to calculate urinary output by this method it is necessary to collect all the urine produced by the person for 24 hours.

In general, it is recommended that the sample collection begins first thing in the morning, discarding the first urine of that day since it corresponds to what was produced during the night.

From the second urination, the urine should be collected in a container of adequate size that can be covered (to avoid evaporation), placing the product of the successive urinations in it until the first urine the next morning, which corresponds to what occurred during the night.

Once this is done, the urinary volume is counted in 24 hours, which is determined in the laboratory with a graduated cylinder.

Once this value is obtained, the calculation is very simple by applying the following formula:

Urinary volume / 24 hours / Body weight

For example, to calculate the urinary output of a person weighing 72 kg and whose urinary volume is 2,800 cc, divide 2,800 by 24 (to know the volume per hour), which gives a value of 116.66 cc / hour

This value is then divided by body weight, that is, 116.66 by 72, which gives a value of 1.6 cc / Kg / hour

The result obtained from the equation is searched in a table to determine whether the urinary output is normal or not.

Direct quantification

For its part, direct quantification is much simpler since the urinary volume collected for one hour is measured in a small graduated cylinder through a urinary catheter connected to a collection bag.

In this case, it is not necessary to wait 24 hours to know the urinary output, in fact it is possible to determine how it varies from hour to hour; To do this, simply empty the contents of the urine collection bag at regular intervals of exactly 60 minutes and measure the amount of urine in the graduated cylinder.

The volume obtained is divided by the weight of the patient and thus the urinary output is obtained, that is:

Urinary volume in one hour / Body weight

For example, to calculate the urinary output of a patient weighing 80 kg from whose urine collector 65 cc are obtained in one hour, divide 65 by 80, obtaining a urinary output value of 0.81 cc / kg / hour.

Normal values

The normal value of urinary output for an adult should be 0.5 to 1 cc / Kg / hour.

When the value of urinary output increases above 3 cc / Kg / hour, we speak of polyurea (increased urinary output).

On the other hand, when the urinary output has a value of 0.3-0.4 cc / Kg / hour, we speak of oliguria (moderate decrease in urinary output), while with figures of 0.2 cc / Kg / hour or less talk of anuria (severe decrease or total absence of urinary output)

References

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2. Burke, DG, Smith-Palmer, T., Holt, LE, Head, B., & Chilibeck, PD (2001). The effect of 7 days of creatine supplementation on 24-hour urinary creatine excretion. Journal of strength and conditioning research, 15 (1), 59-62.
3. Burke, DG, Smith-Palmer, T., Holt, LE, Head, B., & Chilibeck, PD (2001). The effect of 7 days of creatine supplementation on 24-hour urinary creatine excretion. Journal of strength and conditioning research, 15 (1), 59-62.
4. Levey, AS, Coresh, J., Balk, E., Kausz, AT, Levin, A., Steffes, MW,… & Eknoyan, G. (2003). National Kidney Foundation practice guidelines for chronic kidney disease: evaluation, classification, and stratification. Annals of internal medicine, 139 (2), 137-147.
5. Chawla, LS, Eggers, PW, Star, RA, & Kimmel, PL (2014). Acute kidney injury and chronic kidney disease as interconnected syndromes. New England Journal of Medicine, 371 (1), 58-66.