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A urine test strip or dipstick is a basic diagnostic tool used to determine pathology changes in a patient's urine in standard urinalysis.
| + Urine test strip | |
| Leukocytes | |
| Nitrite | |
| Urobilinogen | |
| Protein | |
| pH | |
| Haemoglobin | |
| Specific gravity | |
| Ketone | |
| Bilirubin | |
| Glucose | |
The test strips consist of a ribbon made of plastic or paper of about 5 millimetre wide. Plastic strips have pads impregnated with chemicals that react with the compounds present in urine producing a characteristic colour. For the paper strips the reactants are absorbed directly onto the paper. Paper strips are often specific to a single reaction (e.g. pH measurement), while the strips with pads allow several determinations simultaneously. (2026). 9789500619387, Editorial panamericana. ISBN 9789500619387
There are strips which serve different purposes, such as qualitative strips that only determine if the sample is positive or negative, or there are semi-quantitative ones that in addition to providing a positive or negative reaction also provide an estimation of a quantitative result, in the latter the colour reactions are approximately proportional to the concentration of the substance being tested for in the sample. The reading of the results is carried out by comparing the pad colours with a colour scale provided by the manufacturer, no additional equipment is needed.http://www.seg-social.es/ism/gsanitaria_es/ilustr_capitulo6/cap6_7_analisorina.htm language = Spanish
This type of analysis is very common in the control and monitoring of diabetic patients. The time taken for the appearance of the test results on the strip can vary from a few minutes after the test to 30 after immersion of the strip in the urine (depending on the brand of product being used).
Semi-quantitative values are usually reported as: trace, 1+, 2+, 3+ and 4+; although tests can also be estimated as milligrams per decilitre. Automated readers of test strips also provide results using units from the International System of Units.
An improper technique can produce false results, for example, leukocytes and erythrocytes precipitate at the bottom of the container and may not be detected if the sample is not properly mixed, and in the same way, if an excess of urine remains on the strip after it has been removed from the test sample, may cause the reagents to leak from the pads onto adjacent pads resulting in mixing and distortion of the colours. To ensure that this does not occur it is recommended the edges of the strip are dried on absorbent paper.
The determination of urinary pH has two main objectives, one is diagnostic and the other is therapeutic. On the one hand it provides information regarding the balance between acid and alkali in a patient and allows identification of the substances that are present in the urine in crystalline form. On the other hand, certain illnesses require a patient to keep the pH of their urine within given narrow margins, whether to promote the elimination of chemotherapeutic agents, avoid the precipitation of salts that promote the formation of kidney stones, or in order to facilitate the control of a urinary infection. Regulating diet mainly controls urinary pH, although using medication can also control it. Diets rich in animal proteins tend to produce acidic urine, while diets mainly composed of vegetables tend to produce alkali urine.
Commercial brands measure pH in increments of 0.5 or 1 pH units between pH 5 and 9. In order to differentiate pH in this wide range it is common to use a double pH indicator system comprising methyl red and bromothymol blue. ADW Diabetes. Methyl red produces a colour change from red to yellow in the range of pH 4 to 6 and the bromothymol blue changes from yellow to blue between pH 6 and 9. In the range 5 to 9 the strips show colours that change from orange at pH 5, passing through yellow and green to dark blue at pH 9.
The urine test strip test for specific gravity is based on the change in dissociation constant (pKa) of an anionic polyelectrolyte (poly-(methyl vinyl ether/maleic anhydride)) in an alkali medium that is ionised and releases hydrogen ions in proportion to the number of cations present in the solution. The greater the cation concentration of the urine the more hydrogen ions are released, thereby reducing the pH. The pad also includes bromothymol blue, which measures this change in pH. (2026). 9789500619387, Editorial panamericana. ISBN 9789500619387 It should be remembered that the test strip only measures cation concentration, it is therefore possible that urine with a high concentration of non-ionic solutes (such as glucose or urea) or with high molecular weight compounds (such as the media used to provide radiographic contrast) will yield a result that will be erroneously lower than that measured by densitometry. The colours vary from dark blue with a reading of 1.000 to yellow for a reading of 1.030.
Chemical tests for blood use the pseudoperoxidase activity of hemoglobin to catalyze a reaction between the heme component of both hemoglobin and myoglobin and the chromogen (a substance that acquires colour after a chemical reaction) tetramethylbenzidine to produce an oxidized chromogen, which has a green-blue colour. Reagent strip manufacturers incorporate peroxide, and tetramethylbenzidine, into the blood testing area. Two colour charts are provided that correspond to the reactions that occur with hemoglobinuria, myoglobinuria and hematuria (RBCs). In the presence of free hemoglobin/myoglobin, uniform colour ranging from a negative yellow through green to a strongly positive green-blue appears on the pad. In contrast, intact red blood cells are lysed when they come in contact with the pad, and the liberated hemoglobin produces an isolated reaction that results in a speckled pattern on the pad. Reagent strip tests can detect concentrations as low as five red blood cells per microliter; however, care must be taken when comparing these figures with the actual microscopic values, because the absorbent nature of the pad attracts some of urine. The terms trace, small, moderate, and large (or trace, 1+, 2+, and 3+) are used for reporting.
False-positive reactions due to menstrual contamination may be seen. They also occur if strong oxidizing detergents are present in the specimen container. Vegetable peroxidase and bacterial enzymes, including an Escherichia coli peroxidase, may also cause false-positive reactions. Therefore, sediments containing bacteria should be checked closely for the presence of red blood cells. Traditionally, ascorbic acid (vitamin C) has been associated with false-negative reagent strip reactions for blood. Both Multistix and Chemstrip have modified their reagent strips to reduce this interference to very high levels of ascorbic acid, and Chemstrip overlays the reagent pad with an iodate-impregnated mesh that oxidizes the ascorbic acid prior to its reaching the reaction pad. False-negative reactions can result when urine with a high specific gravity contains crenated red blood cells that do not lyse when they come in contact with the reagent pad. Decreased reactivity may also be seen when formalin is used as a preservative or when the hypertension medication captopril or high concentration of nitrite are present. Red blood cells settle to the bottom of the specimen container, and failure to mix the specimen prior to testing causes a falsely decreased reading.Urinalaysis and Body Fluids Sixth Edition by Susan King Strasinger and Marjorie Schaub Di Lorenzo
Specific kidney and urinary tract diseases that can be identified include: chronic kidney disease, glomerulonephritis, proteinuria and haematuria.
Traditional reagent strip testing for protein uses the principle of the protein error of indicators to produce a visible colorimetric reaction. Contrary to the general belief that indicators produce specific colours in response to particular pH levels, certain indicators change colour in the presence of protein even though the pH of the medium remains constant. This is so because protein accepts hydrogen ions from the indicator. The test is more sensitive to albumin because albumin contains more amino groups to accept the hydrogen ions than other proteins. Depending on the manufacturer, the protein area of the strip contains different chemicals. Multistix contains tetrabromophenol blue and Chemstrip contains 3’,3”,5’,5”-tetrachlorophenol, 3,4,5,6-tetrabromosulfonphthalein. Both contain an acid buffer to maintain the pH at a constant level. At a pH level of 3, both indicators appear yellow in the absence of protein. However, as the protein concentration increases, the colour progresses through various shades of green and finally to blue. Readings are reported in terms of negative, trace, 1+, 2+, 3+ and 4+ or the semi-quantitative values of 30, 100, 300 or 2000 mg/dL corresponding to each colour change. Trace values are considered to be less than 30 mg/dL. Interpretation of trace readings can be difficult.Urinalysis and Body Fluids by Susan King Strasinger and Marjorie Schaub Di Lorenzo
A urine test strip showing positive for blood can also indicate hemoglobinuria, which is not detectable using a microscope due to the lysis of red blood cells in the urinary tract (particularly in alkaline or dilute urine), or intravascular hemolysis. Under normal conditions the formation of haptoglobin-hemoglobin complexes prevents glomerular filtration, but if the hemolysis is extensive haptoglobin's uptake capacity is exceeded and hemoglobin can appear in urine. Hemoglobinuria can be caused by hemolytic anaemia, blood transfusions, extensive , the bite of the recluse spider (Loxosceles), infections and strenuous exercise.
The urine test strip test for blood is based on hemoglobin's pseudo peroxidase activity in catalysing a reaction between hydrogen peroxide and the chromogen tetramethylbenzidine in order to produce a dark blue oxidation product. the resultant colour can vary between green and dark blue depending on the amount of hemoglobin.
The presence of myoglobin in urine gives a positive reaction in the test strip's blood test but the urine appears clear with a red to brown colouration. The presence of myoglobin in place of hemoglobin can be caused by pathologies associated with muscular damage (rhabdomyolysis), such as trauma, crush syndrome, prolonged coma, , progressive muscular atrophy, alcoholism, heroin abuse and strenuous physical activity.
The haem fraction of these proteins is toxic for the kidney tubules and elevated concentrations can cause acute kidney injury.
It is possible to use an ammonia sulphate precipitation test in order to distinguish between hemoglobinuria and myoglobinuria. This consists of adding 2.8gr of ammonia sulphate to 5 mL of centrifuged urine, mixing well and after 5 minutes filtering the sample and centrifuging again. The hemoglobin precipitates out with the ammonia sulphate but not the myoglobin. Analysis of the supernatant for blood with a test strip will give a positive if myoglobin is present and a negative if hemoglobin is present.
The test can give false positives if strong oxidant or peroxide residues are present on the laboratory material used for the analysis.
Around 30–40% of type I diabetics and around 20% of type II diabetics suffer in time from a nephropathy, and early recognition of diabetes is therefore of major significance for the further state of health of these patients.
Specific carbohydrate metabolism disorders able to be identified include diabetes mellitus, glucosuria and ketonuria.
The detection of glucose by test strips is based on the enzymatic reaction of glucose oxidase. This enzyme catalyses the oxidation of glucose by atmospheric oxygen to form gluconic acid and hydrogen peroxide. A second linked reaction, mediated by a peroxidase, catalyses the reaction between the peroxide and a chromogen (a substance that acquires colour after a chemical reaction) to form a coloured compound that indicates the glucose concentration.
The reaction is specific for glucose, as occurs in all enzymatic reactions, but it can provide some false positive results due to the presence of traces of strong oxidising agents or peroxide from disinfectants used on laboratory instruments.
An increase in fat metabolism can be the result of starvation or malabsorption, the inability to metabolize carbohydrates (as occurs, for example, in diabetes) or due to losses from frequent vomiting.
The control of urinary ketone is particularly useful in managing and monitoring diabetes mellitus type 1. Ketonuria indicates an insulin deficiency that indicates the need to regulate its dosage. An increase in the blood concentration of ketone produces a water-electrolyte imbalance, dehydration and if not corrected, acidosis and in the end diabetic coma.
The three ketone compounds appear in different proportions in the urine, although these proportions are relatively constant in different samples as both the acetone and the beta-hydroxybutyric acid are produced from the acetoacetic acid. The proportions are 78% beta-hydroxybutyric acid, 20% acetoacetic acid and 2% acetone.
The test used in the urine test strips is based on the reaction of sodium nitroprusside (nitroferricyanide). In this reaction the acetoacetic acid in an alkali medium reacts with the sodium nitroprusside producing a magenta coloured complex:
The test does not measure beta-hydroxybutyric acid and it is only weakly sensitive to acetone when glycine is added to the reaction. However, as these compounds are derived from the acetoacetic acid their existence can be assumed and a separate test is not therefore necessary. Those medicines that contain sulfhydryl groups, such as mercaptoethane sulphonate Na (Mesna) and captopril and L-DOPA can give atypical colouring. A false negative can occur in samples that have not been adequately stored due to volatilization and bacterial degradation.
Specific liver diseases and haemolytic disorders able to be identified include liver disease, (accompanied by jaundice), cirrhosis, urobilinogenuria and bilirubinuria.
Conjugated bilirubin appears in urine when the normal degradation cycle is altered due to the obstruction of the biliary ducts or when the kidney's functional integrity is damaged. This allows the escape of conjugated bilirubin into the circulation as occurs in hepatitis and hepatic cirrhosis).
The detection of urinary bilirubin is an early indication of liver disease and its presence or absence can be used to determine the causes of clinical jaundice.
The jaundice produced by the accelerated destruction of red blood cells does not produce bilirubinuria, as the high serum bilirubin is found in the unconjugated form and the kidneys are unable to excrete it.
The test strips use a diazotization reaction in order to detect bilirubin. The bilirubin combines with a diazonium salt (2,4-dichloroaniline or 2,6-dichlorobenzene-diazonium-tetrafluoroborate) in an acid medium to produce an azo dye with colouration that varies from pink to violet:
False positive reactions can be due to unusual pigments in the urine (for example, yellowy orange phenazopyridine metabolites, indican and the metabolites of the medicine Lodine (Etodolac)). False negatives can also be given by poorly stored samples as the bilirubin is Photosensitivity and undergoes photo oxidation to biliverdin when it is exposed to light, or hydrolysis of the glucuronide can occur producing free bilirubin which is less reactive.
Any deterioration in liver function reduces its ability to process the recirculated urobilinogen. The excess that remains in the blood is filtered out by the kidneys and appears in urine. When hemolytic disorders occur the amount of unconjugated bilirubin that is present in the blood increases causing an increase in hepatic excretion of conjugated bilirubin, resulting in increased amounts of urobilinogen that in turn causes an increase in reabsorption, recirculation and renal excretion.
The reactions that take place in the test strip vary according to the manufacturer, but in reality there are two reactions that are most frequently used. Some manufacturers use Ehrlich's reaction (1), in which urobilinogen reacts with p-dimethylaminobenzaldehyde (Ehrlich's reagent) in order to produce colours that vary from light to dark pink. Other manufacturers use a diazo coupling reaction (2) that uses 4-methoxybenzene-diazonium-tetrafluoroborate to produce colours that vary from white to pink. The latter reaction is more specific.
A number of substances interfere with the Ehrlich reaction on the Multistix strip: porphobilinogen, indican, p-amino salicylic acid, sulphonamide, methyldopa, procaine and chlorpromazine. The test should be carried out at room temperature as the reaction's sensitivity increases with temperature. Poorly stored samples can yield false negative results as the urobilinogen suffers photo oxidation to urobilin that does not react. The formaldehyde used as a preservative produces false negatives in both reactions.
The reactive strips detect nitrite by using the Griess test in which the nitrite reacts in an acid medium with an aromatic amine (para-arsanilic acid or sulphanilamide) in order to form a diazonium salt that in turn reacts with tetrahydrobenzoquinoline to produce a pink azo dye.
The nitrite test is not particularly reliable and negative results in the presence of clinical symptoms are not uncommon, meaning that the test should not be taken as conclusive. Negative results can be obtained in the presence of non nitrate-reducing microorganisms. Nitrite-reducing bacteria need to remain in contact with nitrate for long enough to produce detectable amounts (first urine produced in the morning or at least with a urine retention of 4 hours). Large numbers of bacteria can react to reduce nitrite to nitrogen, which will give a false negative result. The use of antibiotics will inhibit bacterial metabolism causing negative results even though bacteria are present. In addition some substances such as ascorbic acid will compete with the Greiss reaction giving unrepresentatively low readings.
The test for leukocyte esterase is purely indicative and should not be solely relied on for diagnosis, as it does not replace microscopic or urine culture examinations.
The urine test strip reaction is based on the action of leukocyte esterase in catalysing the hydrolysis of an ester of indolecarboxylic acid. The indoxyl that is liberated combines with a diazonium salt in order to produce a violet coloured azole dye.
The esterase reaction needs about 2 minutes to take place. The presence of strong oxidising agents or formaldehyde can cause false positives. False negative results are associated with elevated concentrations of protein (greater than 500 mg/dL), glucose (greater than 3 g/dL), oxalic acid and ascorbic acid. Urine with a high specific gravity can also cause leukocyte crenation, which can impede the liberation of the esterases.
| Specific Gravity
Reference range
Physiological range | 1.016–1.022
1.002–1.035 Range: 1.000–1.030 | |
| pH value
First morning urine
During the day | 5–6
4.8–7.4 Range: 5–9 | |
| Leukocytes
Reference range
Grey zone | < 10 Leu/μL
10–20 Leu/μL | 10–25 Leu/μL |
| Nitrite | – | 0.05 mg/dL (11 μmol/L) |
| Protein Albumin | < 2 mg/dL | 6 mg/dL |
| Glucose
First morning urine
During the day | < 20 mg/dL
< 30 mg/dL | 40 mg/dL (2.2 mmol/L) |
| Ketones
Acetoacetic acid
Acetone | < 5 mg/dL
– | 5 mg/dL (0.5 mmol/L)
40 mg/dL (7 mmol/L) |
| Urobilinogen | < 1 mg/dL | 0.4 mg/dL (7 μmol/L) |
| Bilirubin | < 0.2 mg/dL | 0.5 mg/dL (9 μmol/L) |
| Blood
Erythrocytes
Haemoglobin | 0–5 Ery/μL
– | 5 Ery/μL
0.03 mg/dL Hb |
It was only towards the end of the 18th century that doctors interested in chemistry turned their attention to the scientific basis of urinalysis and to its use in practical medicine.
Even though the test strips have changed little in appearance since the 1960s, they now contain a number of innovations. New impregnation techniques, more stable colour indicators, and the steady improvement in colour gradation have all contributed to the fact that the use of urine test strips has now become established in clinical and general practice as a reliable diagnostic instrument. The parameter menu offered has steadily grown longer in the intervening decades.
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