Urinalysis vs. urine culture
Q Most labs I have worked in have urinalysis
(chemistry) entirely separate from urine cultures (micro). A few other
labs I have worked at correlate urine-culture growth with the urinalysis
results. For instance, if a culture shows growth that is not compatible
with the urinalysis results (1+-4+), the urine analysis (UA) is repeated
the next day as the UA report is read along with the plates. Also, for
example, while reading plates, if the UA dipstick was reported as
leukocyte esterase and nitrite negative but 2+ bacteria is reported
microscopically, the UA will be repeated (sometimes 24 hours later).
This tends to alter urinalysis results to be compatible with culture
results as well as having dipstick results compatible with microscopic
results. The discussion of urinalysis vs. urine culture has come up in
our lab. Is there support for the practice of correlating or matching
urinalysis results with culture growth?
A I would not advise repeating tests in order to make
results, retrospectively, compatible with subsequent information. Your
query does, however, raise the important subject of the sensitivity and
specificity of urinalysis for detecting urinary-tract infection, and the
correlation between urinalysis and urine culture. In part, the answer to
this question will depend upon the patient population being tested.
Among women with typical symptoms of urinary-tract infection, such as
frequency or dysuria, the presence of pyuria by microscopy or leukocyte
esterase by dipstick both correlate highly with culture-proven bactiuria
(80% to 90%).1
In a general laboratory receiving unscreened samples, the
picture may be more complex, although the same general trends hold. In this
situation, lack of pyuria or dipstick leukocyte esterase makes a positive
urine culture (>105 organisms) unlikely (<1%).2
Furthermore, samples with a pyuria are more likely to contain a reportable
pathogen (64%) than demonstrate subsequent lack of growth (8%).2
The presence of both pyuria and bactiuria is highly specific for the
presence of culturable urinary pathogens, but somewhat less sensitive than
the presence of only one or the other alone.3 The same can be
said for the detection of leukocyte esterase and/or nitrite by dipsticks.
The presence of more than 10 squamous epithelial cells per millimeter3
makes the culture of multiple organisms more likely (53%), and can be used
as a criterion for possible specimen contamination during collection.2
Therefore, urinalysis can be used to screen urine samples
that are likely to prove either culture positive or culture negative.
Incorporation of algorithms to prescreen samples prior to urine culture may
be a cost-effective method to select those likely to yield clinically useful
information.4
It is important to keep in mind, however, that in high
risk patients (e.g., pregnant women with symptoms), even urine samples that
have negative microscopy or dipstick testing should still be cultured, since
in this situation missing even a small percentage of positive samples could
have clinically important consequences.3
—Mykola V. Tsapenko, MD, PhD
—John C. Lieske, MD
Division of Nephrology and Hypertension
Renal Function Laboratory
Mayo Clinic
Rochester, MN
References
- Young JL, Soper DE. Urinalysis and urinary tract
infection: update for clinicians. Infect Dis Obstet Gynecol.
2001;9(4):249-255. - Smith P, Morris A, Reller LB. Predicting urine
culture results by dipstick testing and phase contrast microscopy.
Pathology. 2003;35(2):161-165. - Whiting P, et al. Rapid tests and urine sampling
techniques for the diagnosis of urinary tract infection (UTI) in
children under five years: a systematic review. BMC Pediatr.
2005;5(1):4. - Shaw KN, et al., Screening for urinary tract
infection in infants in the emergency department: which test is best?
Pediatrics. 1998;101(6):E1.
Diluting samples with high CK value
Q I would just like to find out if it is safe to continue
diluting a sample with a high CK value until you get the final result
without any flags from the chemistry analyzer. I was told that enzymes are
diluted only up to a certain point since the enzyme will lose its activity
if overdiluted. Can you kindly give me your technical and medical opinion
about this?
A Enzymes require cofactors and, in some cases, will have
very different activity in matrices other than human blood. Thus, there will
be a limit beyond which dilution will cause an inaccurate estimation of
enzyme activity, unless you are able to dilute into human plasma or blood
with no enzyme activity. Rather than take this approach, a better option is
for each lab to define the clinical reportable range (CRR) for each test
offered based on the analytical measurement range (AMR) and the clinical
relevance of high results.
In the example you gave, let us assume that your
instrument can measure neat plasma up to a CK value of 10,000. Dilution one
hundredfold would allow you to measure CK values up to 1,000,000. Your
medical director must then make a judgment about whether values greater than
1,000,000 have clinical relevance, or whether results reading on your
instrument as “>10,000” even after one hundredfold dilution into an
appropriate diluent will be reported as “>1,000,000.” If the medical
director believes that reporting up to 1,000,000 is appropriate to allow
good medical care in your institution, then the lab must then verify that
one hundredfold dilution of CK values on that instrument using an
appropriate diluent producing accurate results. If so, then the lab has
defined the CRR for the test and validated the dilution protocol. Some labs
choose to define a single maximum dilution for all tests on a given
platform, while others define maximum dilution on a test-by-test basis. Some
regulatory agencies require that the AMR and CRR be defined for every test
offered; so this should be something you consider for every test that can be
diluted to produce a result.
—Brad Karon, MD, PhD
Best way to do body-fluid cultures
Q I am researching the best way to do body fluid cultures —
pleural fluid, ascites fluid, and paracentesis fluid in particular. Is it
acceptable to centrifuge an aliquot 10 mL to 15 mL and utilize the sediment
for culture and Gram stain? Is it acceptable to place the fluid into blood
culture bottles? It seems that some physicians like to submit fluid
specimens in blood culture bottles, but I was told that those bottles were
not validated for blood, not other body fluids. Has this changed over time?
I have also wondered about using a cytospin centrifuge for a Gram stain on
these fluids and then placing aliquots in blood culture bottles. What
suggestions do you have?
A I refer you to “A Guide to Specimen Management in
Clinical Microbiology,” an excellent book on specimen management. Sterile
body fluids, as you indicate, can be processed into blood culture bottles or
submitted to the laboratory in sterile screw-capped containers. A minimum of
1 mL and up to a maximum of 10 mLs (the more the better) should be placed
into an aerobic and an anaerobic blood culture bottle. This would be
acceptable practice when looking for bacteria and yeast and can be done at
the site of the specimen collection or in the laboratory. A separate part of
the specimen should be submitted for Gram stain analysis. Body fluids should
always be microscopically examined after Gram stain of a cytocentrifuged
specimen. Body fluids can also be centrifuged and the sediment processed for
culture as you state, but there is information that submission in blood
culture bottles will increase detection of infection.1,2,3,4,5 In
addition, if you do process specimens after centrifugation, it would be best
to centrifuge as much fluid as possible (up to 50 mLs; again, the more the
better).
What we currently do our in laboratory is ask that body
fluids be inoculated into blood culture bottles at the time of the
collection procedure. We also ask that a small (0.5 mL to1 mL) amount of
direct specimen also accompany the bottles. We use this for a cytospin Gram
stain. If organisms are seen on the specimen Gram stain, then we set solid
media as well from this direct specimen (if no organisms are seen on Gram
stain, no plates are set). We do this because if organisms are seen in the
direct smear, there is a greater likelihood that we might recover them a day
sooner on solid plated media, rather than waiting for the blood culture
bottles to flag as positive and then plating to solid media. Body fluids
with positive organism smears, however, are a small number of our body fluid
specimens.
—Susan E. Sharp, PhD(DABMM)
Director of Microbiology
Kaiser Permanente
Portland, OR
Further reading
Miller JM. A Guide to Specimen Management in Clinical
Microbiology. 2nd ed. Washington, DC: ASM Press; 1999.
References
Lakshmi V. Culture of body fluids using the bact/alert
system. Indian J Med Microbiol. 2001;19(2):44-50.
Azap OK, Timurkaynak F, Sezer S, Cagir U, et al.
Value of automatized blood culture system in the diagnosis of continuous
ambulatory peritoneal dialysis peritinoitis. Transplan Proc.
2006;38(2):411-412.
Bourbeau P, Riley J, Heiter BJ, Master R, et al. Use
of the BacT/Alert blood culture system for culture of sterile body
fluids other than blood. J Clin Microbiol. 1998;36(11):3273-3277.
Pawar GP, Gupta M, Satija VK. Evaluation of culture
techniques for detection of spontaneous bacterial peritonitis in
cirrhotic ascites. Indian J Gastroenterol. 1994;13(4):139-140.
Runyon BA, Canawati HN, Akriviadis EA. Optimization
of ascitic fluid culture technique. Gastroenterology.
1998; 95(5):1351-1355.
Brad S. Karon, MD, PhD, is assistant professor of
laboratory medicine and pathology, and director of the Hospital Clinical
Laboratories, point-of-care testing, and phlebotomy services at Mayo
Clinic in Rochester, MN.
