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Information

Editors

TimoKouri
HeidiAlenius

Urinalysis and Bacterial Culture

Investigations

  • A routine urinalysis consists of chemical analysis (reagent strip or dipstick test) and particle counting (basic count or advanced differential count) performed either by microscopy (urine sediment or chamber counting) or using automated analysers.
    • The sensitivity of a multiple reagent strip test is approximately 20 × 106 cells/l (2-3 cells per field of view) for leucocytes and 10 × 106 cells/l (1-2 cells per field of view) for erythrocytes.
  • A bacterial culture is requested when a urinary tract infection is suspected. In particular cases, special cultures may be agreed on with the local microbiological laboratory in selected patients especially if particle counting is used to screen negative bacterial cultures. In addition, tuberculosis bacilli, chlamydiae and gonococci, among others, do not grow on an ordinary bacterial culture plate (these have their specific test procedures).
  • Urine sample for cytological studies is ordered when a malignant neoplasm is suspected (examined in a pathological laboratory).
    • Urine sample that is as fresh as possible (second sample in the morning, i.e. urine that has been retained in the bladder for 2-3 hours)
    • If the sample cannot be delivered to the laboratory within two hours, it should be centrifuged and the sediment fixed in 50% alcohol.
  • Investigations are chosen according to the suspected illness. Arrangements should be agreed upon locally.

Selection of investigations according to the suspected illness

Suspected urinary tract infection (UTI), bacteriuria and pyuria

  • The diagnosis of recurrent acute cystitis in an otherwise healthy adult female patient should be based on recognised symptoms. Laboratory investigations are not necessary (see picture Urinalysis in the Diagnostics of Urinary Tract Infections). In other cases, a bacterial culture is indicated; see Urinary Tract Infections.
  • A rapid test is used to establish the presence of pyuria and/or bacteriuria either by chemical screening (demonstration of leucocytes and nitrites) or by standard particle counting.
    • The sensitivity of rapid testing in determining bacteriuria in a dysuric patient is only about 50-60% compared with the sensitivity of bacterial culture (nitrite or leucocyte test positive; nitrite test alone only 20-50%), if one considers even a small growth of uropathogens (from 103 colony-forming units [CFU]/ml) in a symptomatic patient as significant.
    • Staphylococcus saprophyticus and certain types of enterococci do not have nitrate reductase. The type of nutrition consumed by the patient or the irritation of the bladder (short bladder time of the urine) do not always favour the production of nitrites in infections caused by other bacteria either.
    • The sensitivity of particle counting as compared to culture is better than that of a reagent strip test. In addition, the result is more precise in automated counting, which is why it is recommended for hospital laboratories that provide emergency services.
    • A high vitamin C intake will give false negative readings for leucocytes in reagent strip testing.
    • Delays in sample transportation, as well as centrifugation, will lead to leucocyte and erythrocyte destruction, resulting in falsely low particle counts.
  • Diagnosing UTI in small children involves verifying the presence of pyuria followed by a bacterial culture of the urine, which is preferably obtained via a bladder puncture . In detection of pyuria, the sensitivity and specificity of chemical analysis is about 80-90% as compared to particle counting.
  • The limits of clinically significant counts of uropathogens and yeasts in urine depend on patient preparation as well as factors associated with the specimen; see table T1. The evaluation of the culture results calls for the consideration of the bladder time (whether the urine had been less than or over 4 hours in the bladder), the method of and success in collecting the sample together with the concentration (relative density, earlier known as specific gravity) of the urine.
  • Even substantial bacterial growth may be due to contamination. A positive result in the bacterial culture should be controlled if
    • the patient is asymptomatic and particle counting is negative (improper transport conditions or even incorrect labelling of the sample referring to wrong patient)
    • the sample contains three or more species of bacteria (improperly collected sample, mixed flora).
  • The aetiology of pyuria may also be other than a microbial infection, for example glomerulonephritis Glomerulonephrites or interstitial cystitis Bladder Pain Syndrome (Interstitial Cystitis).

Clinically significant limits for uropathogen and yeast concentrations. The clinical significance and the amount of bacterial growth may be greatly dependent on the sample collection method and the patient's condition, and hence the limits presented in the table below primarily provide guidance for the ordering of further bacterial cultures.

Clinical presentation or sample typeLimits of significant concentration (colony forming units, CFU/ml)
Asymptomatic bacteriuria
Suspected UTI (no knowledge of the quality of the midstream sample)		105
Catheter sample from a female patient104
Catheter sample from a male patient103
Symptomatic patient with suspected urinary tract infection and a good quality midstream sample*103
Bladder puncture sample, quantitative culture102
Bladder puncture sample, enrichment cultureNo limit
* 103 -104 CFU/ml with 1-2 different species may be a significant growth if the patient is dysuric and the sample collection succeeded properly (E. coli and Staphylococcus saprophyticus).

Suspected haematuria

  • Haematuria Haematuria may be caused by:
    • a systemic illness (for example bleeding diathesis)
    • an illness of the kidneys or lower urinary tract
    • an artefact (menstruation)
    • physical strain (e.g endurance sports).
  • Chemical testing with a reagent strip is sufficient for screening. The method is based on the pseudoperoxidase activity of haem and is clinically adequate for exclusion. The haem from myoglobin is also detected by the strip test (rhabdomyolysis).
  • Identified haematuria should be controlled by particle counting.
  • The upper reference limit for urinary erythrocyte count in healthy individuals depends on diuresis and on the method of analysis; it is approximately 10-20 × 106 /l.

Testing for urinary particles of renal origin

  • A basic urinary particle count will give only a gross estimate of urinary particles of renal origin.
    • Particle count includes the total number of casts and the number of "small epithelial cells" that include the renal tubular epithelial cells and the transitional epithelial cells of the lower urinary tract.
    • In the automatic count, the total amount of epithelial cells includes the squamous epithelial, transitional epithelial and tubular epithelial cells.
  • A nephrological differential count on the urinary sediment will differentiate in more detail between tubular cells and those originating from the transitional epithelium and identify different cast types. In addition, leucocytes (both granulocytes and lymphocytes) and erythrocytes may be of renal origin.
  • The morphology of erythrocytes can be investigated separately if the patient has isolated haematuria. Erythrocytes originating from the kidneys are dysmorphic. Availability of the investigation should be confirmed from the laboratory.
  • The upper reference limits for adult fasting early morning urine (sediment microscopy; in Finnish standardization, 1 field of view = 0.12-0.17 µl depending on the diameter of the ocular's field of view in the microscope):
    • women: 3-4 granulocytes, 1-2 epithelial cells, 1-2 erythrocytes per field of view
    • men: 1-2 granulocytes, 1-2 erythrocytes per field of view
    • no casts should be present.

Suspected proteinuria

  • Renal disease is usually associated with proteinuria Proteinuria.
  • Screening using a conventional multiple reagent strip detects albumin from the concentration 0.2 g/l, i.e. 200 mg/l, upwards.
  • More sensitive methods are required for the detection of microalbuminuria associated with diabetes or hypertension (at least 10 mg/l of albumin should be detected): measured from an overnight collection of urine or from a single early morning sample (urine albumin/creatinine ratio). Sensitive strip tests are also available, if centralized regional quantitative analysis is not compatible with the care situation.
  • The excretion of immunoglobulin light chains in myeloma cannot be detected with a reagent strip test. Myeloma is diagnosed by fractioning of the serum and urine proteins (electrophoresis).

Other basic chemical investigations

  • Detecting an increased concentration of glucose in the urine with reagent strips is particularly useful when out-of-hours diagnosis is needed in ill children or elderly patients. Blood glucose should always be checked if the patient's urine tests positive for glucosuria.
  • Ketone bodies can be detected with the Legal's test of a reagent strip. It will turn positive already after 12 hours of fasting. When monitoring diabetic ketosis at a hospital it is recommended to measure hydroxybutyrate from plasma (available also as a point-of-care test on whole blood), because the Legal's test only reacts to acetoacetic acid and acetone.
  • The pH of the urine is physiologically between 5 and 9. Adjustment of pH may be attempted in some patients with renal stones or when the elimination of certain drugs requires acceleration.
  • The measurement of relative density (earlier specific gravity) provides suggestive information on the concentrating ability of the kidneys:
    • dilute urine: relative density 1.000-1.005
    • isotonic urine: relative density 1.010-1.015
    • concentrated urine: relative density 1.020 or more.
  • Osmolality will provide the best estimate of the concentrating ability. Osmolality of plasma and of isotonic urine is approximately 300 mOsm/kg H2O. Urine osmolality may also be approximated on the basis of plasma creatinine concentration or specific conductivity of the urine.

Choosing the investigations

  • Each test request should be individually considered according to the clinical need.
  • The earlier applied stepwise phasing where the chemical analysis led to particle counting only if the initial chemical screening was positive to erythrocytes, leucocytes or proteins has been mainly discarded along with the technological development.
  • Classification
    • Suspected UTI
      • No investigations (otherwise healthy female patients)
      • Chemical analysis/particle count and bacterial culture (possibly special cultures; other patients).
      • Out-of-hours in primary care: chemical analysis is carried out as a point-of-care test and urine specimens are either cultivated or the specimen is kept in a refrigerator overnight waiting for cultivation the next morning.
      • At a hospital, the selection of investigations is agreed upon with the laboratory; particle count is often available for emergency diagnostics in addition to the other investigations mentioned above.
    • Other indications than suspected UTI
      • Chemical analysis
      • Basic or differential particle count as needed, sometimes erythrocyte morphology

Patient guidance

  • The main problems encountered with urine samples are associated with their collection, the filling and labelling of specimen tubes and the reliability of transport as well as relaying of sufficient patient history details in the electronic information systems. The patient must be given both oral and written instructions (multilingual as needed; local contact details and laboratory opening hours clearly indicated). The division of duties and the procedures concerning patient guidance should be clearly agreed upon in the institutions providing health or other, e.g. social care.

Factors associated with the patient preparation that affect the results of urinalysis

  • Concentration of the urine sample
    • Overnight fasting (if possible to arrange) will ensure sufficient concentration of urine. The patient may drink, at the most, a glass of water in the morning. Only patients who are clearly dehydrated may drink more. A dilute sample will yield false negative results (to be noted when interpreting the results).
    • To reveal even minor abnormalities in the urine composition, a concentrated morning sample should be obtained if the patient's acute symptoms do not prevent this. If the patient presents out-of-hours (bladder irritation, abdominal pain, acute haematuria, etc.), the sample is not timed and the variation in diuresis should be taken into account when interpreting the results (the water excretion of 20-50 ml/h during the night may increase to 200 ml/h or even to 500 ml/h during the day, and the resulting dilution of urine will hinder the detection of particles or proteins or the detection of uropathogens in bacterial culture.)
  • Physical exercise
    • Orthostatic proteinuria and exercise-induced proteinuria may be avoided by collecting a sample in the morning after the night's rest (albumin/creatinine ratio from the first morning urine vs. another sample during the day) if they are not the actual target of the investigation.
  • Bladder time
    • To ensure bacterial growth, the urine should be retained in the bladder for 4-6 hours before sample collection (for E. coli, the duration of the cell cycle is approximately 2 hours).
    • In outpatient care, it may be more practical to use the second concentrated morning sample (collected at the laboratory) than a sample collected at home.
    • Samples that have been in the bladder for a shorter time than four hours have to be accepted from patients with dysuria. This should be taken into account when interpreting the results.
  • Medication
    • The laboratory should be informed of any past or current antimicrobial medication to aid the sensitivity testing of any cultured uropathogens.

Sample collection

  • A midstream urine sample (instructed) will decrease microbiological contamination from the outer genitalia.
    • A sample for chlamydia testing should be a ”first-catch” urine sample (collected no sooner than two hours after last voiding, see ).
  • A sample may be collected from a young child by using a specimen jar that has been attached to the front wall inside the child's pot with ”blue tac” or similar. A sample from an infant may be collected into an adhesive bag or preferably with a specific collection pad. However, such a sample is only reliable in ruling out a UTI.
  • An infant's UTI should be confirmed with a bladder puncture sample (video Urine Aspiration Sampling in a Child). ”In-and-out” catheterization may introduce a bacterial colonization into the child's bladder.
  • The best way to collect sterile samples from patients with permanent catheters is by inserting a new catheter and letting the sample flow through it into the tube. If the sample has to be collected from the tip of the permanent catheter while changing the bag, urine should first be let to flow in waste so that the possible positive result would not merely indicate colonisation of the catheter. From a suprapubic percutaneous catheter, the sample is preferably taken by puncturing the catheter through the point especially constructed for this purpose.
  • In-and-out catheterization is recommended in bladder function disorders and in incontinent elderly patients instead of microbiologically poor samples taken from the bedpan.
  • Special samples are also collected from a cystostomy or during cystoscopy.
  • The method, time and success of sample collection (especially concerning important midstream samples with high risk of contamination), the time of last voiding (more or less than four hours) and possible antimicrobial medication should be indicated in the data system so that the person who interprets the results has the possibility to use the background information when evaluating the findings. It should be borne in mind that especially the culture result will be assessed in relation to the sample quality.

Sample storage and transport

  • A specimen jar (primary collection container) provided by the laboratory is used to collect the urine specimen which is then transferred to transport tubessupplied by the laboratory (home-found jars or tubes should not be used).
    • When only a reagent strip test is performed as a point of care test at the care unit, the tubes are not required.
    • The specimen containers should be shaken before transferring the urine into the tubes and before testing.
    • The handling of bladder puncture specimens should follow local laboratory guidelines.
  • Chemical analysis (strip test) is carried out at the health care facility as a point-of-care test when the sample has reached room temperature (+20°C). This will take at least about 15-30 minutes from sample collection.
  • The sample should be sent to the laboratory using transport tubes with preservatives (allowing room temperature storage and transport), as agreed with the laboratory, or be stored at +4°C (refrigerator or a cool box) until testing, if a preservative is not used (intra-house transport).
  • Reagent strip testing can be performed on a sample without a preservative within 8 hours even if the sample was kept at room temperature, and within 72 hours if kept in a refrigerator.
  • A sample collected for microscopic examination or particle counting (basic or differential count, erythrocyte morphology) should be stored with a preservative due to possible delays in transport and examination.
  • A urine sample should be cultured as soon as possible, because the time to preliminary culture results alone is 18-24 hours (rich bacterial growth becomes discernible in a shorter time, weak growth will emerge during 48 hours). Identification of the bacterial species and antimicrobial sensitivity testing will take an additional 24-72 hours.
    • The sample for bacterial culture will keep in a refrigerator for 24 hours; or 48 hours if kept in preservative containing tubes (for Pseudomonas aeruginosa only 24 hours).
    • A bladder puncture specimen should either be cultivated immediately in a blood culture bottle (enrichment broth) or by dropping a few drops on a chocolate agar plate which is immediately transported to the laboratory for culture. Check the instructions of your laboratory particularly regarding out-of-hours samples.

    References

    • Kouri TT, Gant VA, Fogazzi GB et al. Towards European urinalysis guidelines. Introduction of a project under European Confederation of Laboratory Medicine. Clin Chim Acta 2000;297(1-2):305-11. [PubMed]