Urinalysis Tutorial

Jphillip

Sujets

• Testudo
• Tutoriel
• Glucose
• Acide nitreux
• Syndrome myélodysplasique
• Éléments du groupe 6

Informations

• Publié par: Ummi
• Nombre de lectures: 231
• Langue: English

Extrait

Urinalysis Tutorial

Created by Geoffrey K. Dube, MD, and Robert S. Brown, MD

© 2004, Beth Israel Deaconess Medical Center, Inc.

Introduction

Patients with renal disease may present with a variety of clinical syndromes, including acute and chronic renal failure, gross and microscopic hematuria, and proteinuria. These syndromes may be detected on the basis of clinical findings or may be discovered on routine blood or urine tests performed for other purposes. The history and physical examination are important to help elucidate the cause of the renal disease. Equally important, however, is analysis of a fresh urine specimen, which may help to identify the type of renal disease present. Fewer physicians and physicians-in-training perform urinalysis today, in part due to the presence of automated urinalysis machines which are readily available in clinical laboratories. Understanding how to perform and to interpret the results of a urinalysis are key clinical skills, as the urinalysis is a simple, inexpensive, noninvasive test which can quickly narrow the differential diagnosis and identify what further diagnostic and therapeutic interventions are needed.

This tutorial is intended to serve as an introduction to the urinalysis for medical students and medical housestaff. In addition to the following discussion of the dipstick and the proper way to examine urine, the tutorial contains four distinct modules, each of which can be reached from the main page. These modules cover four common nephrologic syndromes: acute tubular necrosis, the nephritic syndrome, the nephrotic syndrome, and crystalluria. The modules do not need to be read through in any particular order. They can be studied in one sitting, in multiple sittings, or used as a reference guide for when specific clinical scenarios are encountered in the inpatient or outpatient setting.

The urine sediment images in this tutorial are mainly from the collection of the late H. Richard Nesson, MD, supplemented by Robert S. Brown, except as noted in the text.

The Dipstick

The urine dipstick is composed of a plastic stick with several chemical reagents attached to it. Each chemical reagent tests for a different property of the urine. Although the chemical reagents and properties tested may differ based on the brand of dipstick, each dipstick generally tests for the following properties: specific gravity, pH, heme, protein, ketones, glucose, leukocyte esterase, nitrite, and urobilinogen. Although each test on the dipstick provides valuable information, this discussion will focus primarily on the ability of the dipstick to test for the presence of erythrocytes, leukocytes, and protein in the urine.

The dipstick test for protein is positive mainly in the setting of large molecular-weight proteins in the urine, such as albumin. The test detects predominantly albumin, but is not very sensitive. Other proteins, such as immunoglobulin light chains, are not measured by the urine dipstick. The urine dipstick will give a semi-quantitative measure of the degree of proteinuria (0, trace, 1+ or 30 mg/dl, 2+ or 100 mg/dl, 3+ or 300 mg/dl, 4+ or 2,000 mg/dl or greater). However, this measure must be interpreted with caution, as it is dependent on the concentration of the urine. A small amount of protein in a very concentrated urine may read 2-3+, while nephrotic- range proteinuria in a patient with very dilute urine may be measured only as 1-2+. False-positive results for protein may be obtained if there is a highly buffered alkaline urine.

In patients in whom the presence of low molecular- weight proteins in the urine is suspected, a sulfosalicylic acid test should be performed. This test is not performed with the dipstick and has a higher false-positive rate. However, the sulfosalicylic acid test detects all proteins in the urine, rather than just albumin.

The urine dipstick tests for the presence of heme, a component of hemoglobin, and for leukocyte esterase, an enzyme found in leukocytes. The test for heme will be positive if there are erythrocytes in the urine. It will also be positive in the presence of hemoglobinuria or myoglobinuria. False negative results are unusual.

The test for leukocyte esterase will be positive in the setting of pyuria of any etiology. The dipstick test for nitrite is a test for bacteriuria, as many Gram- negative bacilli species are able to convert urinary nitrate to nitrite. The presence of leukocyte esterase in the absence of nitrite suggests the possibility of sterile pyuria, such as may occur with interstitial nephritis, renal tuberculosis and nephrolithiasis. However, only about 80% of urinary tract infections will be nitrite-positive by dipstick.

The dipstick test for urobilinogen will be positive in disease states associated with elevated levels of conjugated bilirubin.

The dipstick test for glucose will be positive when elevated serum glucose levels cause spillage of glucose into the urine to a degree that overwhelms the kidney’s ability to reabsorb filtered glucose; this state does not usually occur until the serum glucose concentration is > 180 mg/dl. The presence of glycosuria in the absence of hyperglycemia suggests proximal tubular dysfunction, such as occurs in the Fanconi syndrome or renal glucosuria.

The dipstick test for urinary pH reflects the degree of acidification of the urine. Its main use occurs in the diagnostic evaluation of metabolic acidosis or nephrolithiasis.

The specific gravity measures the concentration of the urine. It generally reflects the urine osmolality. However, in the presence of dense molecules such as glucose or radiocontrast media, the specific gravity will be elevated out of proportion to the urine osmolality.

Ketones form in the setting of insulin deficiency, when increased lipolysis leads to increased free fatty acid metabolism, and ultimately, to the production of the ketoacids, acetone, acetoacetate, and beta- hydroxybutyrate. The latter is not detected by the dipstick. Ketoacids form in the setting of diabetic ketoacidosis, alcoholic ketoacidosis, and fasting ketoacidosis.