Physicochemical properties of urine

Color

The normal color of urine changes from light yellow to dark brown, and is determined by a pigment called “Urochrome” and how dilute or concentrated the urine is¹.
Pigments and other compounds presented in certain foods and medications can change the color of urine. An unusual urine color may denote a disease². For example, dark red or brown urine is a characteristic of hematuria, the presence of blood in urine. The normal color of urine changes with the amount of water ingested. Liquids dilute the yellow pigments in urine, so the more you drink, the clearer is the urine. When you drink less, the color becomes more concentrated³.

Odor

Most changes in urine odor go away with time, since in most cases it is not a disease. Some foods and medications affect the odor of urine. If foul odors are present in the urine, it may be due to bacteria. Sweet urine odor may also be a sign of diabetes or a rare metabolic disease³.

pH

Usually pH is determined by means of a test strip. It measures the concentration of free protons in the urine. Therefore, it is a partial assessment of acidification of the distal tubule. The range of values is very wide (4.5-8), depending on the acid-base status of the body. After a fasting situation, it usually has a low or acid value, whereas after a postprandial situation, it usually has a high or basic value, especially in vegetarian diets⁴.

Volume

Urine water losses usually represent the major water loss in sedentary adults. Even so, urine volume can vary considerably, from about 500mL to several liters per day. This is mainly influenced by age and weight. After that, it will depend especially on hydration, nutrition, cardiovascular and renal status of the patient and associated pathologies⁵.
Polyuria: Excretion of urine in quantities greater than 2.5 liters. The reason may be physiological, due to ingestion of too much liquid or diuretics such as coffee, tea or beer. It can also be pathological, due to diabetes⁶.
Oliguria: It is the emission of urine volume lower than normal. To be considered as oliguria, the volume must be less than 400ml. It can be caused by a decrease in glomerular filtration, an increase in tubular reabsorption or an obstruction of the lower urinary tract. It can also be caused by increased vomiting, diarrhea or severe burns³.
Anuria: It is extreme oliguria. Urine there is no urine excretion (rarely) or less than 100 ml. It may be due to renal obstruction caused by kidney stones or tumors³.
Nocturia: Excessive or uncontrolled nocturnal urine excretion. Optimal treatment should focus on lifestyle modification (e.g., reducing fluid intake in the evening). For patients with sustained discomfort, medical therapies should be introduced; low-dose desmopressin has been shown to be effective. Diuretics should be taken mid-afternoon, depending on the individual. Patients who do not respond to these basic treatments should be referred for specialized treatment⁷.

Density

Urine specific gravity measures the concentration of particles in urine and the density of urine compared to water. Normal results range from 1010 to 1020. Abnormal results are usually those below 1.010 or above 1.020 in patients with certain kidney diseases⁸.

Osmolarity

It is a more accurate measure of concentration than density. The reason is that osmolarity is a value that depends only on the concentration of its solutes. Density, on the other hand, also depends on the nature of the solutes. If these solutes include proteins or carbohydrates, the density is higher. This is not the case with osmolarity. Normal osmolarity values are 300-800 mOsm/kg. The interpretation of the results is the same as for density⁹.

Bibliography

1. Martínez-Ávila MC, Rodríguez Yánez T, Borré Naranjo D, Almanza A, Dueñas Castell C. ¿Es la bolsa o la orina lo que cambió de color? Diagnóstico diferencial y relevancia clínica. Medicina Clínica Práctica. 2021 Oct 1;4(4):100274.
2. Padilla Cuadra JI. ¿Cómo interpretar un examen general de orina? 2018 Jul;
3. King Strasinger S, Schaub Di Lorenzo M. Análisis de orina y de los liquidos corporales. 5a ed. Ed. Médica Panamericana, editor. 2010. 41–52 p.
4. Tortora GJ, Bryan D. Principios de anatomía y fisiología, 13va Edición. 13th ed. Editorial Medica Panamericana Sa, editor. México D.F.: Editorial Medica Panamericana Sa; 2013. 1067–1099 p.
5. Agua e hidratación: Bases fisiológicas en Adultos. 2017.
6. Ramírez-Guerrero G, Müller-Ortiz H, Pedreros-Rosales C. Poliuria en el adulto. Una aproximación diagnóstica basada en la fisiopatología. Revista Clínica Española. 2021 May 28;
7. Oelke M, de Wachter S, Drake MJ, Giannantoni A, Kirby M, Orme S, et al. A practical approach to the management of nocturia. International Journal of Clinical Practice. 2017 Nov 1;71(11).
8. Flasar C. ¿Qué es la densidad específica de la orina? Nursing (Ed española). 2009 Aug;27(7):52.
9. Osmolality (Urine) – Health Encyclopedia – University of Rochester Medical Center. (n.d.). Retrieved October 11, 2022, from https://www.urmc.rochester.edu/encyclopedia/content.aspx?contenttypeid=167&contentid=osmolality_urine