Liquid gold part 2 - Why and how to filter the golden information to maximize the value of the microscopic examination of urine

Introduction

Complete urinalysis should be performed every time a need to evaluate the renal system exists. However, the complete urinalysis is not only of value for evaluation of the renal system. The urinalysis can provide information about other body organ systems like the liver as well as help characterize underlying systemic acid-base abnormalities, the severity of diabetes and diabetic ketoacidosis as well as assisting in characterizing various hematologic abnormalities like hemolytic disease.

Routine urinalysis can easily and accurately be performed in a veterinary practice. There are different components of a complete urinalysis. The physical examination of the urine collected includes crude but important gross inspection of the urine. The chemical examination primarily consists of multiple tests performed on the classic dry reagent strip testing commonly known as the “Dip Stick” test. The microscopic examination consists of evaluation of the formed elements of a urine specimen and is something that should be performed in a short time following the collection of the urine specimen.

Urinalysis - microscopic examanation

Microscopic examination of urine sediments requires the availability of a good quality microscope. If one is not available accurate assessment of the formed elements of the urine will not be possible. Both a good 10x and 40x objective is required on the microscope. When examining the urine sediment microscopically, most people use a bright field microscope that is used for routine hematology and cytology specimens. When examining the urine specimen, putting the light out of bright field microscopy settings by lowering the substage condenser and partially closing the iris on the microscope may prove helpful in making various formed elements easier to visualize. The preparation of the sediment requires only a simple tabletop centrifuge with a relatively slow speed to allow sedimentation of the formed elements without mechanical damage to these elements. Most people examine the sediment on unstained preparations, but there are a series of commonly used stains including some developed specifically for urine sediment examination (Sedi Stain) or routine Romanovsky stains commonly used in hematology or cytology specimen analysis (Diff Quik, Wright’s stain, etc.). The initial examination of an unstained specimen will be difficult. However, learning how to read these specimens precludes the observation of stain precipitate and mis-identifying this as an organism or crystal and prevents the bacterial or fungal contamination of the stains with subsequent overgrowth and misinterpretation of the presence of these organisms. Practice with some pseudo urine specimens by using a clean urine specimen spiked with various “formed elements” commonly seen in urine sediment specimens. Epithelial cells from the buccal mucosa (spatula gentle scraping of the oral cavity), leukocytes and erythrocytes from anticoagulated whole blood originally submitted for hematology evaluation, and potential neoplastic or various inflammatory cell populations from other specimens (fine needle aspirates of solid masses, sediment smears of effusions, etc.). Some of the more commonly encountered formed elements in urine are briefly discussed below.

Cells

Erythrocytes

Red blood cells in the urine, hematuria, can originate from any region of the urinary tract but most commonly is associated with local bleeding in the urinary bladder associated with cystitis or it is iatrogenic during the collection process. In fresh urine, the cells are non-nucleated and close examination reveals their biconcave shape. In dilute urine, erythrocytes swell and lyse resulting in hemoglobinuria rather than hematuria. In concentrated urine, erythrocytes typically crenate and resemble crenated erythrocytes in the peripheral blood with multiple short uniform sharp cytoplasmic projections. Practice in the identification of these cells in urine is relatively simple. Place a drop of EDTA anticoagulated blood into dilute, non-concentrated and concentrated urine samples and see the effect on the erythrocytes.

Leukocytes

White blood cells are commonly seen with cystitis or any other type of inflammatory disease in the urinary tract. Typically only very few leukocytes are seen in normal urine (less than 2 WBC / HPF). These cells are usually spherical and in contrast to the erythrocytes, nuclei are visualized with careful inspection at 40x objective magnification field of view. Neutrophils are the most common leukocyte seen in urine and the lobed nucleus is usually very easy to identify even in unstained preparations. Mononuclear leukocytes, monocytes, macrophages and lymphocytes, have rounded to slightly indented nuclei and are slightly larger than the neutrophil. If there is ever a question regarding the identification of leukocytes or recognizing what type of leukocyte is present, preparation of a concentrated sediment air-dried specimen of the urine sediment and staining with a stain you use for hematology is recommended. This will allow direct detailed visualization of nuclear and cytoplasmic morphology similar to evaluation of a blood film.

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