What is a urinalysis (urine test)?
Urine test has been used as a diagnostic tool since the 2nd Century CE. It is a painless and simple to collect test that provides quick results, which makes it much less inconvenient than blood tests, which can only be made through use of needles.
A complete urinalysis may give us important tips about possible diseases, especially urinary tract problems. The presence of blood, pus, proteins, glucose, and several other substances in the urine is usually an important tip for diseases that may not yet be showing clear signs or symptoms.
The fact that urine is of normal appearance does not mean absence of changes. Even the presence of blood may be only microscopic, meaning that it is impossible to identify through any means other than laboratory testing of the urine.
Urine can also be used to check for the presence of licit or illicit drugs in the body. However, for this type of research, special examinations need to be requested. The urinalysis does not intend to be a urinary drug screening.
The four most common types of urine tests are:
- 24-hour Urine Collection.
- Urine cultures.
- Urinary drug screening
In this article, we shall restrict our analysis to the simple urine test known as urinalysis.
The information contained herein is to help you understand the results of urinalysis. The patient must refrain from using this text for the interpretation of urine tests without guidance from a doctor. The presence of leukocytes in the urine, a change in pH, a description of epithelial cells, the presence of mucus, or any other findings shown in the urinalysis must always be related to the medical history, the symptoms, and the physical examination of the patient.
Urinalysis is the most simple of all urine tests, and is made by collecting between 40 and 50 mL of urine in a small plastic pot. We normally recommend collecting a sample of urine “mid-stream”, which means not collecting the first or last part of urine that comes out. This reduces the risk of the sample being contaminated with bacteria or impurities that could be present in the urethra (the urinary canal that brings the urine from the bladder).
The first urine in the morning is normally used, but this practice is not mandatory. Indeed, the urine may be collected at any time of day.
The urine sample shall ideally be collected in the laboratory itself, as the fresher the urine, the more reliable the results. An interval of over two hours between collection and appraisal could render the results invalid, especially if the urine has not been kept under refrigeration.
The urinalysis is divided into two parts. The first involves chemical reactions, while the second part entails viewing drops of urine through a microscope.
In the first part of the test, a special dipstick is immersed in the urine, as can be seen in the picture at the start of the text. Each strip has several coloured squares with chemicals that react with certain elements in the urine. This part of the test is so simple that it can be carried out in the doctor’s surgery. After one minute, the colour of the square is compared to a reference table that is normally given on the package of the dipsticks for urinalysis.
Through these reactions, and with microscopic examination, it is possible for us to detect the presence, and the quantity, of the following data for the urine supplied:
- Red Blood Cells.
- Leukocytes (white blood cells).
- Urobilinogen and Bilirubin.
- Epithelial cells and cylinders.
The results shown by the dipstick are qualitative rather than quantitative, which means that the dipstick identifies the presence of the substances mentioned above, but the quantification is approximate. The result is normally shown in a scale of crosses, from 1 to 4. For example, a urine sample with a reading of “proteins 4+” has a large content of proteins, while urine with “proteins 1+” has a small protein content. When the concentration is very small, some laboratories present the result as “traces of protein”.
Urinalysis: how to interpret results
The specific gravity of pure water equals 1000. The closer the specific gravity of urine is to this value, the more diluted it is. Normal values range from 1005 to 1035. Urine samples with specific gravity (SG) close to 1005 are well diluted; those close to 1035 are highly concentrated, which could suggest dehydration. Urine samples with density close to 1035 are normally deep yellow in colour and tend to have strong odour.
The specific gravity typically shows the concentration of the solid substances diluted in the urine, mainly mineral salts. The less water there is in the urine, the higher its density or SG shall be.
Urine is naturally acidic, as the kidney is the main means of elimination of acids from the body. While the typical pH of blood is usually in the region of 7.4, in the case of urine the pH may vary from 5.5 to 7.0, which is much more acidic.
When the urine has a pH of 7 or higher, this could suggest the presence of bacteria that alkalinise the urine. Other factors that could make the urine more alkaline are: a diet poor in animal protein; a diet rich in citrus fruits or dairy products; and the use of medication such as acetazolamide, potassium citrate, or sodium bicarbonate. Having thrown up some hours prior to the test may also be a cause of excessively alkaline urine. In rare cases, some disorders of the renal tubules can also make the urine show a pH of more than 7.0.
When the pH of the urine is less than 5.5, this could be a sign of acidosis in the blood or disease in the renal tubules. A diet with a high level of animal protein could also make the urine more acidic. Other situations that increase the acidity of the urine include fits of diarrhoea or the use of diuretics like hydrocholorothiazide or chlortalidone.
The most common value for pH of urine is between 5.5 and 6.5; however, values above or below this reference range do not necessarily mean the presence of some illness. This result should be interpreted by your doctor.
All the glucose filtered in the kidneys is reabsorbed by the blood, using the renal tubules. This means that the normal result is an absence of glucose in the urine.
The presence of glucose in the urine is a strong sign that the levels of glucose in the blood are high. It is very common for diabetics to show a loss of glucose through the urine. This occurs because the level of sugar in the blood is so high that part of this sugar ends up eliminated by the urine. When the levels of blood glucose are higher than 180 mg/dL, there is normally a loss of urine.
The presence of glucose in the urine, in people who do not have diabetes mellitus, is normally a sign of diseased renal tubules. This means that, even though there is not excessive glucose in the urine, the kidneys are not available to prevent its loss.
Briefly, the presence of glucose in the urine shows an excess of glucose in the blood, or kidneys with tubular dysfunction.
Most proteins circulating in the blood are too big to be filtered by the kidneys and for this reason, under normal conditions, it is not common to identify the presence of proteins in the urine. In fact, there could even be small quantities of proteins in the urine, but these are so few that they are not normally detected by the dipstick test. Therefore, normal urine, which is the urine of a healthy kidney, does not have any protein.
Small amounts of protein in the urine can have dozens of different causes, ranging from benign and trivial causes, like presence of fever, physical exercise some hours before the test, dehydration, or emotional stress, through to more serious causes such as urinary infection, lupus, glomerular diseases and renal lesions caused by diabetes.
Larger quantities of protein in the urine, on the other hand, nearly always suggest the presence of kidney disease, specifically of renal glomeruli, which are the microscopic structures responsible for the filtering of the blood. When the quantity of lost proteins is very high, the patient normally has frothy urine and often has swellings all over the body, which is a clinical situation known as nephrotic syndrome.
There are two different ways of showing the results of the tests for protein in the urine: using a scale of crosses or by means of an estimated quantification in mg/dL.
Form using crosses:
- Lack of protein (normal value).
- Traces of protein.
Form shown in mg/dL:
- Less than 10 mg/dL (normal value).
- Between 10 and 30 mg/dL.
- 30 mg/dl.
- 40 to 100 mg/dL.
- 150 to 350 mg/dL.
- Over 500 mg/dL.
The presence of proteins in the urine is known as proteinuria and shall always be investigated. The 24-hour urine examination is normally performed to make an exact quantification of the total quantity of protein being lost through the urine.
Red blood cells in the urine
As in the case of proteins, the total presence of red blood cells in the urine is usually negligible, and cannot be detected using the dipstick test. Once again, the results are normally shown using a scale of crosses. The normal situation is a lack of red blood cells (haemoglobin).
As the red blood cells are indeed cells, they can be viewed through a microscope. In this way, together with the dipstick test, we can also look for red blood cells directly through the microscope, a technique known by the name of sedimentoscopy. Through the microscope, we can detect any presence of blood, even minimum quantities that cannot be detected using the dipstick method.
In this case, the normal values can be described in two separate ways:
- Less than 3 to 5 blood cells per field or less than 10,000 cells per mL.
The presence of blood in the urine is known as haematuria and can occur with several diseases, such as infections, kidney stones, and serious renal disease.
A false positive result may occur in women who collect urine during their menstrual periods. In this case, the blood detected does not come from their urine, but rather from residual blood that is still present in the vagina. In men, the presence of semen in the urine can also bring a false positive result.
Once haematuria is detected, the next step is to appraise the shape of the blood cells, through a special examination called “erythrocyte dysmorphism”. The dysmorphic red blood cells are those with different shapes, which are common in some diseases such as glomerulonephritis. It is possible that there could be small quantities of dysmorphic blood cells in the urine without this having any clinical relevance. Only values above 40% to 50% are normally considered relevant.
Not all laboratories have skilled personnel to carry out this test. For this reason, on many occasions it is not automatically performed. The doctor needs to make a specific request for this appraisal.
Leukocytes or pyocytes – Leukocyte Esterase
Leukocytes, also known as pyocytes, are white blood cells, the cells that defend us. The presence of leukocytes in the urine normally means that there is some inflammation in the urinary tract. In general, it suggests urinary infection, but they could be present in many other situations, including trauma, use of irritating substances, or any other inflammation not caused by an infecting agent. We can make things simpler, and say that leukocytes in the urine means the presence of pus in the urine.
As they are also cells, leukocytes may also be counted with the use of sedimentoscopy. Normal values are below 10,000 cells per mL or 5 cells per field.
Some dipsticks have a special square for the detection of leukocytes, and normally the result is shown as “leukocyte esterase”. The normal result is negative.
Ketones or ketone bodies
Ketone bodies are products of the metabolism of fats. Ketone bodies are produced when the body is finding difficulty to use glucose as a source of energy. The most common causes are diabetes, prolonged fasting, and strict diets. Other less common situations include fever, acute disease, hyperthyroidism, pregnancy, and even breast-feeding.
The production of ketones is normally very low, and these are not present in the urine.
Some types of medication, including captopril, valproic acid, vitamin C (ascorbic acid) and levodopa can lead to false positives.
Urobilinogen and bilirubin
Both also normally absent in urine samples, they can suggest liver disease or haemolysis (abnormal destruction of red blood cells). Bilirubin only starts appearing in the urine when its levels in the blood exceed 1.5 mg/dL. Urobilinogen can be present in small quantities without this having any clinical importance.
Urine is rich in nitrates. The presence of bacteria in urine turns these nitrates into nitrites. Therefore, a strip with positive nitrites is an indirect sign of the presence of bacteria. Not all bacteria can metabolise nitrates, and for this reason if a urine test is negative for nitrites, even this does not rule out the possibility of a urinary infection.
In fact, the AES test only suggests infection. The presence of red blood cells, together with leukocytes, and a positive result for nitrites, speaks in favour of a urinary infection, but the only way to be sure is to have a urine culture test.
The presence of nitrites is confirmed through the Griess reaction, which is the name given to the reaction between nitrites and an acid medium. For this reason, some laboratories give the result as Griess positive or Griess negative, which just mean positive or negative for nitrites, respectively.
This is probably the most misinterpreted result, both by patients and by some doctors. The presence of crystals in the urine, especially calcium oxalate, calcium phosphate or amorphous urates, is of no clinical importance. Different from what one may think, the presence of crystals does not suggest a greater propensity to the formation of kidney stones. However, it is important to mention that, in some specific cases, the presence of certain types of crystals could be a sign of some disease;
The crystals that do indeed have clinical relevance are:
- Cystine crystals: These suggest an illness called cystinuria.
- Magnesium ammonium phosphate crystals (also known as struvite or triple phosphate crystals): These could be normal, but are also present in cases where the urine is excessively alkaline, a situation caused when there is infection by the bacteria of the genera Proteus or Klebsiella. Patients with struvite kidney stones normally have these crystals in the urine.
- Tyrosine crystals: These are present in an illness called tyrosinaemia.
- Bilirubin crystals: These crystals normally suggest liver disease.
- Cholesterol crystals: These are usually a sign of massive loss of protein through the urine.
The presence of uric acid crystals, if in large quantities, should also be considered as important, as these could arise in patients with gout or neoplasms, such as lymphoma or leukaemia. However, in small quantities, uric acid crystals are quite common and do not suggest any problems.
Epithelial cells and casts
The presence of epithelial cells in the urine is quite normal, and it is the very cells in the urinary tract that peel off. They are only clinically important when they cluster together in the form of a cast, in which case they receive the special name of epithelial cast.
As the renal tubules are cylindrical, whenever we have any substance (protein, cells, blood…) in large quantities in the urine, they cluster together ito forma a cast. The presence of such casts suggests that this substance has come from renal tubules, and not from any other points of the urinary tract, such as the bladder, ureter, prostate and so on. This is very important in cases of bleeding, where a haematic cast suggests that the glomerulus is the origin, rather than the bladder, for example.
The casts that could suggest some health problem are the following:
- Haematic cast (blood): Suggest glomerulonephritis.
- White blood cell casts: Indication of kidney inflammation.
- Epithelial cast: Suggest lesions to the tubules.
- Fatty cast: Suggest proteinuria.
Hyaline casts do not suggest any disease as such, but could be a sign of dehydration.
The presence of mucus in the urine is not specific, and normally occurs through the accumulation of epithelial cells with crystals and leukocytes. It is of very low clinical significance, more a matter of mere observation.
With regard to the urinalysis , it is important to stress that this is an analysis that always warrants interpretation. False positives and negatives are very common, and it is not possible to reach any diagnosis just by comparing the results obtained with the reference results mentioned.
Ascorbic acid in the urine
It is very common for laboratories to call attention to the fact whenever there is ascorbic acid (vitamin C) in the urine. This is very important because ascorbic acid may change the results obtained in the dipstick tests, especially in the detection of elements such as haemoglobin, glucose, nitrites, bilirubin and ketones. It is therefore important for the doctor to know that unexpected results could be false positives or false negatives caused by vitamin C.
Example of a normal urinalysis
By way of example, we now present a model of how laboratories present the results of a summary urine test. This urine examination is normal.
COLOUR — citrine yellow
APPEARANCE — clear
DENSITY — 1.015 (the range of normal values is from 1005 to 1030)
PH — 5.0 (normal readings are between 5.5 and 7.5)
Urine dipstick chemical analysis
Glucose — absent
Proteins — absent
Ketones — absent
Bilirubin — absent
Urobilinogen — absent
Leukocytes — absent
Haemoglobin — absent
Nitrites — negative
Epithelial cells — some
Leukocytes — 5 per field
RBCs — 3 per field
Mucus — absent
Bacteria — absent
Crystals — absent
Cylinders — absent
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