URINARY CONCENTRATING ABILITY IN PREGNANT WOMEN WITH ASYMPTOMATIC BACTERIURIA*

Abstract

Asymptomatic bacteriuria, defined as the presence of over 100,000 pathogens per ml repeatedly in clean-voided urine specimens, occurs in about 4% of pregnant women. Pregnant women with significant bacteriuria cannot be distinguished from a control group without bacteriuria with respect to symptoms, physical examination, or routine urinalysis, but have a higher incidence of the development of acute pyelonephritis of pregnancy. Renal function studies indicate that pregnant women with asymptomatic bacteriuria may have impairment of urinary concentrating ability. Following 24 hours fluid deprivation the mean urine osmolality for 20 women with asymptomatic bacteriuria was 706 mOsm. per kg. H2O compared with a mean maximal urine osmolality of 935 mOsm per kg H2O for 30 comparable women without bacteriuria (P< 0.01). Serum osmolality and creatinine concentrations were the same in both groups. Endogenous creatinine clearances averaged about 20% lower in the bacteriuria group. Rates of solute excretion and osmolal clearance averaged about 9.5% higher in the bacteriuric group. Serial studies in a few women post partum indicate that impairment in concentrating ability improves when bacteriuria clears and persists when bacteriuria remains. Acute pyelonephritis develops more commonly in bacteriuric pregnant women with impairment of concentrating ability than in women with bacteriuria and normal concentrating ability. These findings suggest the possible presence of silent, active pyelonephritis in these women and the need for long term follow-up. Direct involvement of the renal concentrating mechanism by infection, an increase in the hydro-nephrosis of pregnancy due to infection and malfunction of the ureters, and changes in the concentrating mechanism secondary to slight reductions in glomerular filtration rate may be important factors leading to the observed impairment of urinary concentrating ability.