Cylindruria

Clin Chem Lab Med 2015; aop
Review
Alberta Caleffi* and Giuseppe Lippi
Cylindruria
DOI 10.1515/cclm-2015-0480
Received March 30, 2015; accepted May 18, 2015
Abstract: The morphological analysis of urine sediment
is an essential part of urinalysis and casts are important
elements of urinary sediment. Their shape is typically
cylindrical, with extremities often rounded. Casts form
within the renal tubules and are made of Tamm-Horsfall
glycoprotein (THG). Under some physiological or pathological conditions THG fibrils aggregate giving rise to
casts, whose formation is favoured by a number of factors including high urine osmolality and/or low pH. Casts
can be found in normal subjects, in non-renal conditions,
such as fever, dehydration, and acute heart failure as well
as in virtually all renal diseases. Casts can be classified
on the basis of their morphology as hyaline, granular,
waxy, fatty, cellular (leukocytic, erythrocytic, epithelial),
containing crystals or microorganisms, pigmented and
mixed. As the correct identification of casts is crucial for
an accurate and timely diagnosis of renal disorders, laboratory professionals should be trained to identify and
classify casts properly.
Keywords: Tamm-Horsfall glycoprotein;
urinary casts; urinary sediment.
urinalysis;
Introduction
Casts are important constituents of urine sediment, as
they can be associated with a number of renal disorders
whose diagnosis may also depends on the correct identification of urinary casts.
In this paper we describe the preanalytical and analytical factors influencing the formation and identification
*Corresponding author: Dr. Alberta Caleffi, U.O. Diagnostica
Ematochimica, Azienda Ospedaliero-Universitaria di Parma,
Via Gramsci, 14, 43126 Parma, Italy, Phone: +39 0521 703764,
Fax: +39 0521 703791, E-mail: acaleffi@ao.pr.it
Giuseppe Lippi: Laboratory of Clinical Chemistry and Hematology,
Academic Hospital of Parma, Parma, Italy
of casts, the mechanisms involved in cast formation, and
their classification and clinical meaning. Special attention
is given to granular, waxy, erythrocytic and renal tubular
cell containing casts for their relevant clinical importance.
Preanalytical and analytical factors
influencing the formation and
­identification of casts
The morphological analysis of urine sediment is an essential part of urinalysis, since it represents a crucial tool
for diagnosing kidney and urinary tract disorders [1, 2].
Although many international and national guidelines
provide a good support to laboratory staff for obtaining the best reliable results by using an appropriate and
standardised methodology [3–5], several preanalytical
issues related to collection, transportation and storage
of urine samples pose serious challenges for the quality
of urinalysis [6, 7]. The accurate identification and classification of urinary sediment particles including casts
requires a high quality specimen, which is represented
by the second void morning sample. This specimen better
preserves the integrity of casts, thus limiting the chance
of lysis or degeneration due to the prolonged permanence
of urine in the bladder during the night rest. Midstream
urine collection is another important preanalytical factor,
as it ensures that the sample is free from mucus or other
contaminants originating from the urethra and external
areas of the urinary tract and genitals [8–10].
It is also important to inform the patient that strenuous physical exercise, such as jogging, performed in the
hours preceding urine collection, may cause urinary
changes including haematuria and heavy cylindruria [7].
As to the analytical phase, it is not advisable to
examine strongly alkaline urine, as a high pH prevents the
formation of casts and favours the lysis of other cellular elements. Additional conditions that may impair the accurate
identification and classification of casts include sample
contamination by genital secretions, the presence of large
amounts of amorphous phosphates and crystals, as well as
urine with low urinary specific gravity or low osmolality [7].
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Caleffi and Lippi: Cylindruria
Emphasis should also be placed on procedures for
sample transportation and storage. The storage of urine
samples for > 3 h may cause lysis and degeneration of casts
and other cellular elements [11].
The quality of the analytical phase of urine sediment
examination is also crucial for generating reliable data,
and should entail the use of appropriate technology and
instrumentation. During recent years, analytical platforms
have been made available for supporting the laboratory
personnel. These analysis platforms include automated
urine sediment analysers based on flow cytometry, digital
image capture systems or automated intelligence microscopy, even though manual microscopy with low and high
magnification still remains the gold standard for this test.
According to international guidelines, the use of phase
contrast microscopy and polarised light is still the best
approach for identification of casts and their morphological details [3, 4].
Formation of casts
Casts are particles with a cylindrical shape with usually
rounded extremities, which originate in the ascending limb of Henle’s loop, distal tubules and collecting
ducts of the kidney and are made of a specific matrix, i.e.
Tamm-Horsfall glycoprotein (THG) or uromodulin. This
is secreted by tubular cells of the thick ascending limb
of Henle’s loop and has a fibrillar structure, fibrils being
unbranched and with a diameter of 9–15 nm.
Interestingly, THG is the most important constituent
of the so-called physiological proteinuria, even though
its role is not entirely clear (a number of potential actions
have been identified, including prevention of urinary
infection and urolithiasis) [12–14].
Under some circumstances, THG fibres aggregate
which causes the formation of cylinders, which take the
shape of tubular lumen where they are formed. The aggregation of fibres is favoured by several factors including
increased intra-tubular concentration of ultrafiltered proteins, low intra-tubular pH and high osmolality, a fact
that explains why casts tend to be uncommon, or in low
numbers, in diluted and alkaline urine.
Classification and clinical
­associations of casts
Casts have a wide spectrum of shapes, sizes and morphologies which depend on the types of particles embedded
into the cast matrix. All this justifies the need for a classification of casts, which is based on both their morphology
and the particles they contain (Table 1).
It is of the utmost importance to remember that since
casts form in the renal tubules all elements embedded into
the matrix of casts derive from the kidneys, a fact which is
of considerable diagnostic importance [7].
Hyaline casts
These casts are composed only of THG, a fact which
explains their low refractive index. Hence, their microscopic identification in bright field microscopy is challenging, and they can be more efficiently detected using
phase contrast microscopy. Hyaline casts may display
a spectrum of morphologies, which includes “fluffy”,
compact, convoluted or wrinkled casts.
Hyaline casts can be found in normal subjects, in paraphysiological conditions, such as after ­strenuous physical
exercise, and in non-renal disorders such as fever, dehydration, acute congestive heart failure or in association
with the use of Henle’s loop diuretics. However, hyaline
Table 1: Classification of casts (modified from [7]).
Type
Subtype
Hyaline
Granular
–
Finely granular
Coarsely granular
–
–
Leukocytic
Erythrocytic
Epithelial (containing renal tubular
epithelial cells)
Haemoglobinic
Myoglobinic
Bilirubinic
Bacterial
Candidal
According to the type of crystals
within the cast
Hyaline-granular (the most frequent)
Granular-cellular
Granular-fatty
Waxy-granular
Waxy-cellular
Etc.
Waxy
Fatty
Cellular
Pigmented
Containing microorganisms
Containing crystals
Mixed
Some authors also include “broad casts”, which in the past were
also known as “real failure casts”. These casts have an increased
diameter, which is due to their formation within renal tubules which
are dilated as a consequence of the chronic renal disease.
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Caleffi and Lippi: Cylindruria
casts can also be present, in variable amounts, in all renal
diseases, including glomerulonephritides, where they
have been found in 100% of the patients investigated [15],
and acute interstitial nephritis, in which they have been
observed in 86% of cases [16]. Most importantly, whilst
in the normal subjects and in the other non-renal conditions mentioned above, hyaline casts are the only urinary
finding, in renal disorders hyaline casts are almost invariably associated with other types of casts and urine sediment particles [15, 16].
Granular casts
These casts have a surface composed of granules, which
can vary in size (Figure 1). The granules can be rather
heterogeneous, ranging from fine (finely granular cast)
up to coarse (coarsely granular cast), dark, clear, and pigmented. It has been demonstrated that in patients with
proteinuria, fine granules contain ultrafiltered proteins
which have been reabsorbed by tubular cells [17], whilst in
non-proteinuric patients coarse granules probably derive
from the degeneration of cellular elements, such as leukocytes and renal epithelial cells present in the tubular
lumen during cast formation [18]. The presence of granular casts always reflects renal injury, and recent studies
on patients with acute kidney injury have demonstrated
that they are, together with renal tubular epithelial cells
(RTEC) and RTEC-containing casts, a sensitive marker of
acute tubular necrosis [19–21].
Figure 1: A finely granular cast (phase contrast microscope, ­original
magnification 400 × ).
By courtesy of Dr. G.B. Fogazzi.
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Waxy casts
These casts typically display a melted wax (waxy) appearance (Figure 2), which confers them a high refractive
index. They are frequently dark, with blunt extremities,
indented and cracked edges and a large size, which is
often several times that of other types of casts.
Interestingly, their composition remains poorly
understood. In one recent prospective study on waxy casts
in patients with glomerulonephritis it has been reported
that THG is not found in these casts [22], a fact which,
however, would need confirmation by others studies and
further investigation.
Also the clinical associations of waxy casts are poorly
known, because those reported in atlases and textbooks
(e.g. renal function impairment either acute or chronic,
renal amyloidosis, etc.) do not have any sound support
from literature. To date, the only sound investigation on
waxy casts is the prospective study mentioned above [22],
in which the urine sediment of 287 patients with different types of glomerulonephritis was evaluated in a standardised way by phase contrast microscopy. Waxy casts
were found in only 29 patients (13.6%), a frequency which
was much lower than that of all the other types of casts.
A statistically high frequency of waxy casts was found
in acute post-infectious glomerulonephritis and in renal
­amyloidosis (44.5% for either condition), whilst it was
low in idiopathic membranous nephropathy (6.0%) and
nil in focal and segmental glomerulosclerosis. Patients
with waxy casts, compared with those without, had
Figure 2: A waxy cast with typical indentend edges and irregular
surface (phase contrast microscope, original magnification 400 × ).
By courtesy of Dr. G.B. Fogazzi.
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Caleffi and Lippi: Cylindruria
significantly higher levels of serum creatinine (159 vs. 97
μmol/L, p < 0.0001) and the presence of waxy casts correlated significantly with granular casts and leukocytic
casts, whilst there was no association with hyaline casts.
Fatty casts
These casts may contain lipid droplets, oval fat bodies
or cholesterol crystals, and are often associated with the
free forms of these elements. Their identification may
require the use of polarised light microscopy, under which
fatty particles embedded into the cast matrix appear as
“Maltese crosses”. The presence of fatty casts in urine is
associated with heavy proteinuria as it is found in patients
with nephrotic syndrome.
Figure 3: An erythrocyte cast with packed erythrocytes (phase
contrast microscope, original magnification 400 × ).
By courtesy of Dr. G.B. Fogazzi.
Cellular casts
These casts include all cylinders containing any type of
cells which may be present in renal tubules such as leukocytes, erythrocytes and RTEC. Therefore, the classification
of cellular casts encompasses leukocytic, erythrocytic,
and RTEC casts.
Leukocytic casts contain different amounts of leukocytes which at times, due to particular physico-chemical features of the urine, can hardly be distinguished
from RTEC. Also in such cases the use of phase contrast
microscopy can facilitate the correct identification of the
particles.
In patients with urinary tract infection, the finding
of leukocyte casts suggests the involvement of the renal
parenchyma, a fact which is important from the clinical standpoint. It is a common view that leukocyte casts
are found in patients with acute interstitial nephritis [22,
23], whilst it is less know that they can also be found in
patients with glomerular diseases [15].
Erythrocytic casts may contain different amounts of
red blood cells (Figure 3). In some cases the number of
erythrocytes is high and the presence of the matrix can be
no longer distinguished.
The presence of erythrocyte casts is considered
a marker of glomerular hematuria, especially when
they are associated with free dysmorphic erythrocytes
in the urine. Erythrocyte casts are found in most glomerular diseases, especially in those with glomerular
cell proliferation (the so-called proliferative glomerulonephritides) [15], although their frequency is highly
variable among studies [7]. However, in contrast with
the current view, erythrocyte casts can also be found
in patients with acute interstitial nephritis (AIN), as it
has been shown in a recent retrospective study on 21
patients with AIN due to different causes, six of whom
had erythrocytic cylindruria (28.5%). It is hypothesised that in such cases the formation of erythrocyte
casts might be the consequence of interstitial vessels
injury secondary to interstitial cell inflammation, with
extravasation of erythrocytes into the interstitial space
and subsequent passage within the tubular lumens
through tubular basement membranes breaks, which in
AIN are very common [16].
Renal tubular epithelial cells casts (epithelial casts)
may contain various amounts of tubular cells (Figure 4),
and are often accompanied by free renal tubular cells,
which ultimately help identifying this type of cast.
Some challenges may emerge in their accurate identification, especially when the cells are degenerated and
are hardly distinguishable from leucocytes. For such
cases phase contrast microscope or supravital stains are
recommended.
The epithelial casts can be found in a number of renal
diseases including glomerulonephritis [15] and acute
interstitial nephritis [16]. In a pilot study published a few
years ago [19], it was demonstrated that a cast scoring
index (CSI) based on the combined amount of RTEC casts
and granular casts in patients with acute kidney injury
correlated with the renal outcome, the CSI being significantly higher in patients who did not recover renal
function.
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Myoglobin casts have a reddish brown colour but are
usually not associated with red blood cells or erythrocyte
casts. They can be observed in the urine of patients with
acute kidney injury associated with severe muscle damage
leading to rhabdomyolysis.
Bilirubin casts typically display the yellow-brown hue
of bilirubin. They can be observed in patients with liver
disease and/or conditions characterised by bilirubinuria [7].
Mixed casts. A large variety of mixed casts can be
found in the urine, some of which are listed in Table 1.
Among them hyaline-granular casts are by far the commonest. In fact, they have been found in 100% of 100
patients with different types of glomerulonephrits [15]
and in 17 out 21 patients with AIN due to different causes
(81%) [16].
Figure 4: A renal tubular epithelial cell containing cast. Please note
the renal tubular epithelial cell free in the urine (phase contrast
microscope, original magnification 400 × ).
By courtesy of Dr. G.B. Fogazzi.
Casts containing crystals or microorganisms
Virtually any type of crystals may be found in casts, those
made of calcium oxalate, either mono- or bihydrated,
being the commonest ones.
These casts clearly indicate that crystals have precipitated within the tubular lumen, a finding which may
be useful in the diagnosis of crystalluric forms of acute
kidney injury, such as acute urate nephropathy.
Casts containing microorganisms can be caused by
infections of renal tissue. Bacterial casts can be difficult to
identify and can be distinguished from other types of casts
using phase contrast microscopy.
The presence of casts containing Candida is suggestive
for a systemic candidiasis with visceral involvement [7].
Pigmented casts
Pigmented casts have peculiar colour which reveals the
presence of pigments derived from the breakdown of cells
or pigmented molecules.
Haemoglobin casts originate from erythrocytes, have a
typical hue that varies between brown and red, and have
often a granular aspect. They are usually found in association with free red blood cells and erythrocyte casts in
patients with renal bleeding from various causes. In rare
instances haemoglobin casts are due to haemoglobinuria
as it may occur in disorders associated with intravascular
haemolysis.
Conclusions
The casts are considered one the effective urinary sediment markers for diagnosing kidney disorders. The origin,
composition, morphology and clinical associations have
been clearly defined for many of them, whereas less
information is available for others (i.e. waxy casts). Some
interesting findings have emerged from recent studies,
especially on the value of granular and RTEC casts in
identifying patients with acute kidney injury associated
with acute tubular necrosis, the possible presence of
erythrocyte casts in patients with AIN and the frequency
and clinical associations of waxy casts in patients with
glomerulonephritis.
The scientific development in the investigation of
urine corpuscular elements over the past 10 years have
been mainly driven by the commercialisation of automated analysers using different technologies, such as
automated microscopy in bright field, flow cytometry and
digital image capture systems. The use of these tools is
highly recommended for the standardisation, accuracy
and efficiency of urinalysis. However, the overall accuracy of these instruments in identifying casts does need
further development, and for the time being the best identification of cast still rests on traditional morphological
analysis.
As the identification of casts is crucial for the accurate
and timely diagnosis of renal disorders, laboratory professionals should be educated to accurately recognise and
classify casts during analysis of urine sediment. The laboratory staff should also be specifically trained in the use
of the standard methodology for identification and classification of casts, including bright field, phase contrast or
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Caleffi and Lippi: Cylindruria
polarised light microscopy, as well as to the use of specific
cytological staining. Therefore, it is essential that clinical
laboratories should be involved in reliable internal quality
control (IQC) programs and external quality assessment
(EQA) schemes for both the preanalytical and analytical
phase of urinalysis [24–26]. The laboratories should be
directly connected with international and national societies or working groups such as the European Federation of
Laboratory Medicine (EFLM) or the Italian intersocietary
SIBioC/SIMeL working group for urinalysis or Italian urinalysis working group (GIAU) [27]. The members of these
groups promote scientific updates, multicenter studies,
meetings, conferences and other educational activities on
urinalysis. Our ultimate goal is to enhance competency in
morphological and clinical analysis of corpuscular elements in urine to produce updated, efficient, and straightforward laboratory reports in connection with the clinical
needs of the patient according to the modern concept of
personalised medicine [28].
Author contributions: All the authors have accepted
responsibility for the entire content of this submitted
manuscript and approved submission.
Financial support: None declared.
Employment or leadership: None declared.
Honorarium: None declared.
Competing interests: The funding organisation(s) played
no role in the study design; in the collection, analysis, and
interpretation of data; in the writing of the report; or in the
decision to submit the report for publication.
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