Essentials of Clinical Nephrology (Shorouk Press, Cairo, 2000, ISBN ...

FeNa = UNa X v ÷ Ucr X v = UNa X v XPcr = UNa X Pcr
PNa Pcr PNa X V X Ucr PNa X Ucr
i.e. we need only to measure plasma, urinary Na and creatinine. Urine
volume is not required. Normal FeNa is less than 0.1.
B. MICROBIOLOGICAL EXAMINATION OF URINE:
In cases of urinary tract infection, urine specimens are examined for
identification of bacteria as well as for its sensitivity to antibiotics by culture
techniques. Taking a proper urine sample is mandatory to avoid false results.
A midstream urine sample is required i.e. when the bladder is full, the
first 200 c.c. is passed to clean the urethra. Then, 10 c.c. is taken in a sterile
container from the urine stream. In the male, glans penis should be cleaned
by sterile water, and in the female the vulva is cleaned properly and during
micturition labia are held away by fingers. In neonates and young children
suprapubic aspiration of urine by fine needle is safe. The presence of more
than one type of organisms in culture, usually means contamination rather
than infection.
When prostatic infection is suspected, three specimen technique of
Stamey is followed. VB1 is the first voided 10 ml of urine (which represents
urethral bacterial flora). VB2 is the midstream urine (which represents the
bladder flora). After that the patient stops micturition and the prostate is firmly
massaged and any discharge comes out is cultured. Then, the patient passes
another 10 ml of urine (VB3) which represents prostatic flora.
In urine culture, a bacterial count of 100,000 or more is needed for the
diagnosis of urinary tract infection. However, a smaller number may be
considered significant in pregnant women, children, and immunosuppressed
patients. When there is pyuria; and urine culture is persistently negative (at
least three cultures should be done), this is called sterile pyuria. In this
condition, urine should be examined by specific types of cultures e.g.
Lowenstein-Jensen media for Mycobacterium; anaerobic culture for
anaerobes; human blood agar for Gradnerella vaginalis; urea plasma agar for
ureaplasmas; irradiated McCoy cells for chlamydia. If the cultures are still
negative, we are mostly dealing with either viral infection or non-infectious
causes of pyuria e.g. foreign body, malignancy or immunologic disease as
SLE or kidney graft rejection.