Urinary tract infections (UTI) 1

Urinary tract infections can be defined as an infection that affects the upper (ureters and kidneys) and lower (bladder and urethra) urinary tracts. The Health Protection Agency (HPA) stated in 2009 that Urinary tract infections is the second largest healthcare associated infection and estimated that it causes up to 20% of all hospital acquired infections.

Urine microscopy is a method used to identify and quantify cells, bacteria and other materials such as casts in urine samples. This may include the use of an inverted microscope or automated system. Sysmex UF1000i automated analyser is in use for urine microscopy in my laboratory and manual microscopy is rarely performed on urines sample received. This is only done when a request specifically asks for cast cells as the UF1000i cannot differentiate them.  In addition, high risk urine samples are usually processed in Category 3 room without any microscopy performed.

Inverted microscope is a semi quantitative method of urine microscopy. The technique involves the use of micro-titre plate with the lamp or light source and condenser located at the top. The objectives, nose-piece and eye pieces are located below the stage. An aliquot (60 µl) is pipette from a well mixed urine sample and dispensed in the microtitre plate. The plate is then left to settle for 5 minutes and then examined within 15 minutes of dispensing under the inverted microscope using x20 objective lens. The number of RBCs and WBCs per urine sample is counted in two fields and the total number of cells reported as number/mm³. The microscopic results are then reported as <40/ mm³, 40-100/mm³, 100-200/mm³ and >200/mm³. This method is also useful for detecting casts in urine samples. It is also used on samples that cannot be processed using the automated analyser such as samples with thick pus, frank blood and requests for Trichomonas. There could be errors in the use of inverted microscope which includes volume discrepancies and uneven distribution of cells.

Alternatively, a drop of urine is placed on a microscope slide and examined under a normal light microscope using x40 objective lens and the results reported as follows

< 2 cells per hpf        -           < 50/mm³

2 - 5 cells per hpf     -           50 - 100/mm³

5 - 10 cells per hpf   -           100 - 200/mm³

> 10 cells per hpf     -           > 200/mm³

Occasionally, it might be necessary to dilute a urine sample for microscopy if the number of cells present is so large that it masked other cells. Urine samples with numerous white cells are double diluted in saline depending on the number of cells present and then multiply the result by 2. Urine samples with numerous red cells are double diluted in white cell diluting fluid to lyse any red cells present and then multiply the result by 2.

The significance of cells, (other than white cells) casts and pathological crystals that are found in urine are detailed below

Bacteria/Yeast cells 

When there is presence of >1000 bacteria cells per ml in a given urine sample, it usually indicates UTI. This is then followed up by culture. However, differentiation should be made between infection and contamination as normal skin flora could highlight a positive result. These cells if not treated could cause kidney infection by ascending through ureters. Yeast cells are reported as present or not present.

Red blood cells (RBC)

The presence of RBC in a urine sample does not necessarily indicate infection as small amount can be seen in healthy individuals and contamination could occur from menstruation in women and can be visibly detected, by automated urine analyser or with the use of a dipstick. However, in other individuals, high level of RBC in urine known as haematuria may indicate infection especially when the WBC is also elevated. Haematuria can also be seen in renal calculi, malignancy, trauma, stones and glomerulonephritis.

Epithelial cells

The presence of squamous epithelial cells in a urine sample usually indicate contamination from the perianal region.

Cast cells 

Cast cells which are cylindrical proteinaceous structures produced by the kidney and usually present and helpful in the diagnosis of renal infection. They are usually formed in the distal convoluted tubule and collecting ducts of nephrons present in the kidney where they then dislodge and can be seen in the urine. There are acellular and cellular cast cells.

Acelluler cast cells

-       Hyaline casts: Hyaline casts are cylindrical, clear and have a low refractive index. They are produced when Tamm-Horsfall protein aggregates and are usually seen in healthy patients, vigorous exercise or dehydration. 

-       Granular casts: Granular cast occurs as result of either degeneration or breakdown of cellular casts and appears like cigar-shaped, fine or coarse inclusions and with higher refractive index than Hyaline casts. They can be seen in chronic renal disease or after strenuous exercise. 

Cellular cast cells

-    RBC casts: The presence of RBC casts usually indicate glomerular bleeding or damage which can occur in glomerulonephritis. They can also be seen in renal infarction and subacute bacterial endocarditis. They appear as a cylindrical shape with or without ragged edges and yellow-brownish colour.

-   WBC casts: The presence of WBC casts usually indicate kidney infection or inflammation such as in acute pyelonephritis and acute allergic interstitial nephritis.

-      Epithelial cell cast: Epithelial cell casts usually occur when there is damage to the tubular epithelium and can be seen in acute tubular necrosis and when a toxic substance such as mercury is ingested.

Pathological crystal cells

The precipitation of crystals of uric acid, calcium phosphates,Calcium oxalate, triple 

phosphate and amorphous phosphate or urates in urine is called Crystalluria. This 

may be normal, asymptomatic or in association with the formation of urinary tract 

calculi. Crystals may form as a result of drug metabolism and can be seen in 

association with the pathological disease as seen in the presence of uric acid 

crystals in the urine of patient with chronic renal disease and gout. Cysteine crystals 

are rarely seen and may indicate an underlying disease. Bacterial infections caused 

by urea splitting bacteria such as Proteus spp. has been linked with the formation of 

urine stones (Ford 2010).

References

Ford M (2010). Fundamentals of Biomedical Science; Medical Microbiology. Oxford
University Press, London.

Health Protection Agency (2009). Trends in rates of Healthcare Associated Infection 
in England 2004 – 2008. Reports for the National Audit Office. London.