Formation of Casts
Urinary casts are formed in the distal convoluted tubule (DCT) or the collecting duct. All casts have mucoprotein known as Tamm-Horsfall protein as a basic component which is secreted by the distal loop of Henle, the distal tubule and the collecting ducts at a fairly constant rate.
Urinary casts can be divided into two main categories : Acellular and Cellular Casts.
[A] Acellular Casts
 Hyaline Cast
 Granular cast
 Waxy Casts
 Fatty Casts
[B] Cellular Cast
 Red Blood Cell Casts
 White Blood Cell Casts
 Bacterial Casts
 Epithelial Cell Casts
[A] Acellular Casts
 Hyaline Cast
Appearance of Hyaline cast in H&E sections of kidney biosy - It appears eosinophillic. See Image Below which shows Numerous Hyaline Cast (Arrow marked)
Most common Type of cast = Hyaline casts are the most common type of casts which are composed of solidified Tamm-Horsfall mucoprotein.
They have smooth texture and a refractive index very close to that of the surrounding fluid, parallel sides with clear margins and blunted ends.
Found in : Hyaline casts can be seen even in healthy patients. They may be seen in increased numbers during dehydration, exercise or diuretic medicines.
www.pgmeeuploads.comImportant Point = "Fractured Hyaline Cast" is characteristic in Multiple Myeloma. The filtered light chains may cause intrarenal obstruction in the distal nephron by co-aggregating with the carbohydrate moiety of Tamm-Horsfall glycoprotein, which is produced in the thick ascending limb of the loop of Henle and form Hyaline casts. Due to presence of Light chains they are brittle and appear fractured. See the H&E sections shown above.
 Granular Cast
Granular casts result from the degeneration of cellular casts, or direct aggregation of plasma proteins or immunoglobulin light chains. They have a textured appearance which ranges from fine to coarse in character.
They are seen after sternous exercise, chronic renal diseases, acute tubular necrosis.
Important Q Point = Muddy Brown Granular cast in a dirty background is Pathognomonic for Acute Tubular Necrosis (ATN) [see image below] www.pgmeeuploads.com
 Waxy Casts
Waxy casts represent the final stage of degeneration of cellular casts. [See Image Below]
They are uaually seen in tubular injury of a more chronic nature than granular or cellular casts like severe chronic renal disease.
These casts are also called renal failure casts. [Note Broad waxy casts are charecteristic of Chronic Renal failure] [See Image Below]www.pgmeeuploads.com
[B] Cellular Casts
A cellular cast may be composed of any of the cells found in the urine sediment, such as RBC, WBC, or renal tubular epithelial cell. The cellular cast appears to result from a clumping of cells that are incorporated in a protein matrix.
 RBC Cast
Red blood cells may stick together and form red blood cell casts. Such casts are indicative of glomerulonephritis, with leakage of RBC’s from glomeruli, or severe tubular damage. See Image below - It appears as clumps of RBC in tubular shape.
Remember = Presence of RBC cast is strongly suggestive of Glomerulonephritis. www.pgmeeuploads.com
 WBC Cast
White blood cells (generally neutrophils) are present within or upon casts. These casts are typical for acute pyelonephritis, but they may also be present with glomerulonephritis. They may also be seen in acute interstitial nephritis, lupus nephritis, and acute papillary necrosis. See Image Below which looks like Some WBC's are embeded in Protein matrix.
Note that No of WBC can be More as in image below.
 Renal Tubular Epithelial Cast
These casts are composed of renal epithelial cells. These casts are seen in conditions such as renal tubular necrosis, viral disease (such as CMV nephritis), and kidney transplant rejection.
See Image Below = Note that it looks like cells are embeded and these cells are larger than WBC with round nucleus. Copare it with WBC cast as you can confuse it.
Also Remember = Polyoma interstitial nephritis includes an early infection of collecting ducts followed by cytolytic changes and tubular destruction. Evidence of tubular injury in the form of apoptosis, cell drop out, desquamation can be seen. These desquamated cells having the intranuclear viral inclusions, better described as “Decoy cells” can be easily seen on urine cytology.
Decoy Cells = clue to post-transplant polyoma virus infection. www.pgmeeuploads.com