The Aging Urinary Tract and Asymptomatic Bacteriuria
Aging is associated with a decline in function of many body systems, including the urinary tract. Bacteria seem to gain an advantage within the urinary tract of the elderly, and it is not uncommon to find bacteriuria; however, not all of these patients have a true urinary tract infection (UTI). UTI is considered to be the most common infection among the elderly1-3 and the most common cause of bacteremia,4 but patients with bacteriuria who can clearly communicate the absence of urinary tract symptoms have asymptomatic bacteriuria (ASB) and do not require treatment. The prevalence of ASB increases with age, and although ASB does not directly affect the health of elderly patients, failure to recognize it as an entity separate from UTI will impact geriatric patients’ health.
Definition of Asymptomatic Bacteriuria
Bacteriuria is simply the presence of bacteria in the urine. Traditionally, UTI has been considered confirmed when the patient has a positive urine culture. Growth of bacteria in a urine culture demonstrates the presence of bacteriuria and does not give a complete picture of the patients’ health status. Those with clearly identifiable symptoms arising from the urinary tract, such as dysuria, urinary frequency, or flank pain, have a UTI. If urinary tract symptoms are absent in the elderly patient with bacteriuria, then ASB must be considered in the differential diagnosis. More specifically, ASB is the finding of 100,000 colony-forming units per milliliter (cfu/mL) of a single organism in two consecutive urine cultures obtained by clean-catch from a woman without urinary tract symptoms.5 For an asymptomatic man, the same is true except that only one positive culture is required from a voided specimen to meet the definition.5 A catheter-obtained specimen from asymptomatic men or women is ASB if 100 cfu/mL of a single organism is isolated in the urine culture.5 A significant proportion of the elderly with bacteriuria have ASB and not a UTI when using these definitions.
Prevalence of Asymptomatic Bacteriuria
The rate of bacteriuria occurrence increases 1-2% per decade of life,4 and the percentage of these with ASB also increases with age. Postmenopausal women age 50-70 have an ASB prevalence rate of 2.8-8.6%,5 while younger women have an ASB prevalence of 1-2%.6 When all geriatric age groups are combined, the prevalence of ASB in men and women living in the community is 3.6-19% and 10.8-16%, respectively.5 Additionally, men over the age of 80 have a prevalence rate of 20%.7 Older individuals more commonly have ASB.
The higher prevalence of ASB in the elderly correlates with general functional decline, as well as illnesses common to this group.4,6-10 It is, therefore, not surprising that ASB is more frequently present in nursing home residents. In a group of ambulatory elderly women living in a geriatric community, ASB was shown to be related to the level of care they required.8 The prevalence in the women who lived in a more independent housing arrangement was 11%, but within the same community those who lived in an apartment building or a nursing home had an ASB prevalence of 18% and 25%, respectively. Eight percent of patients in nursing homes develop bacteriuria every 6 months,6 and the estimated prevalence of ASB is 25-50% for women and 15-40% for men who are institutionalized.5 Aging as well as functional decline contribute to ASB prevalence.
Pathophysiology of Asymptomatic Bacteriuria
The cause of ASB in the elderly is multifactorial, but incomplete emptying of the bladder is believed to be the primary source.4,6,7,9,11 Bacteria gain access to the urinary tract by the ascending route from the perineum and are typically eliminated by urine flow. Impairment in this defense mechanism is the primary etiology of ASB in the elderly. The remaining paragraphs of this section will review problems common to the elderly that contribute either directly or indirectly to bladder and urinary tract dysfunction, including menopause. Patients with anatomic abnormalities, chronic indwelling catheters, and spinal cord injuries are excluded from this discussion.
Changes within the vagina from menopause and estrogen deficiency promote colonization of the introitus by bacteria from the colon, thereby providing easier access to the urinary tract for bacteria. Aging—and not just the altered vaginal environment—may, however, be the most important risk factor in postmenopausal women; menopause disappeared as a risk factor when age adjustments were applied to data in a study of women with diabetes12 and a group of women with incontinence living in the nursing home.13 Additionally, estrogen replacement therapy (ERT) has not consistently demonstrated a benefit in reducing bacteriuria. Women with incontinence in nursing homes did not have a reduction in bacteriuria or pyuria,14 and UTI occurrence was not reduced15 after oral ERT. Hextall et al13 commented that only two of five randomized, placebo-controlled trials have demonstrated that estrogen replacement is beneficial for reducing bacteriuria; one of the two trials evaluated estrogen cream use. In contrast, a different study actually suggested a borderline increase in the UTI rate with use of estrogen cream.16 Menopause may contribute to ASB among elderly women, but the evidence does not support a recommendation for ERT.
There are several problems in geriatric patients that directly contribute to improper emptying of the bladder, and consequently ASB. Bladder dysfunction occurs with estrogen deficiency13 and urinary incontinence,6,17 two common findings in elderly women. Also, a high postvoid residual (PVR) implies incomplete emptying of the bladder and has been found to be a risk factor for ASB4,6; however, ASB does occur without a high PVR.18,19 Other disorders common to the elderly that may impair bladder function directly or by limiting the patient’s general function and toileting behaviors (eg, stroke, parkinsonism, dementia) have been identified as potential causes.4,5,7,9,10 While it would seem intuitive that medications with the potential to alter bladder function might increase the occurrence of ASB, this has not conclusively been shown to be true.20,21 Aging causes ASB by directly affecting the ability of the bladder to contract, and because the elderly often have other illnesses that alter bladder function.
Some aging changes that are unrelated to bladder function may result in ASB as well. Bowel incontinence is not necessarily associated with urinary tract dysfunction, but repeated soiling of the perineum increases the exposure of the urinary tract to bacteria.6 A decline in kidney function may also have a role. With aging, there is reduced ability of the kidney to concentrate the urine, and a decrease in acidification of the urine.6,7 Although it is more likely that ASB is related to prostate problems because of an obstructive phenomenon, bacteriuria defenses also decrease with the decline in secretions from the prostate.6,9 Finally, immune system decline with aging could allow ASB development.3,4,6, The origin of ASB is multifactorial, but the consensus remains that incomplete bladder emptying is the primary cause.
Aging and neurogenic bladder dysfunction may account for the higher prevalence of ASB in persons with diabetes. Duration of type 1 diabetes correlates with ASB development, but in persons with type 2 diabetes, age and a history of previous urinary infections were identified as risk factors.22 Metabolic control of diabetes has not consistently demonstrated a direct correlation with ASB.9,12,13,23-26 While the origin of ASB is likely multifactorial in patients with diabetes, age is also a predisposing risk.
Asymptomatic Bacteriuria Resembles Urinary Tract Infection on Urine Testing
ASB masquerades as a true UTI. Their shared similarities make it difficult to differentiate the two entities by urine testing alone; therefore, each is best identified by the presence or absence of specific urinary tract symptoms. By definition, both ASB and UTI patients have a positive urine culture, and they also both result in an abnormal urinalysis.
ASB and UTI provoke an immune system response, making urine testing unreliable for a definitive diagnosis. ASB is associated with increased serum and urinary markers of inflammation,9,27,28 including pyuria. Pyuria is less predictive for UTI with aging,17 and even more so in populations with prevalent ASB. White blood cells are present in the urine of 78% of persons with diabetes who have ASB,9 and as many as 90% of elderly nursing home residents have pyuria with or without bacteriuria.6,9,28 In an evaluation of women with incontinence in the nursing home, the positive predictive value of pyuria for UTI was 56% and the negative predictive value was 69%.29 Additionally, a positive urine nitrite test indicates the presence of bacteria; however, this test alone, or with pyuria, is also not sufficient for initiating therapy in the elderly. A meta-analysis of urine dipstick testing demonstrated that the absence of pyuria along with negative nitrite testing are effective in ruling out bacteriuria in all age groups, but positive tests need to be confirmed by other methods in the elderly.30 ASB contributes to this inaccuracy. An abnormal urinalysis does not prove the patient has a UTI; specific symptoms are the most reliable indicator.
Deciding whether a patient has ASB or a UTI can be difficult in some elderly individuals. A UTI can present with a nonspecific clinical change in the elderly, such as altered mental status, and the severely debilitated patient may not be capable of communicating symptoms clearly. This group cannot be precisely categorized as having UTI or ASB based on the definitions of each. If, however, elderly patients can communicate effectively and do not have specific urinary tract symptoms, then they will likely not have UTI, even when abnormal urine tests exist. For example, a nonspecific finding such as fever, along with a positive urine culture, is not a good predictor of UTI in nursing home residents without urinary tract symptoms (positive predictive value of 12%).6 Abnormal urine tests and nonspecific clinical changes are more likely not the result of UTI in populations with prevalent ASB. Physicians may elect to treat the elderly patient who is severely ill and cannot articulate the presence or absence of specific urinary tract symptoms, but finding an abnormal urinalysis or a positive urine culture should not end the search for another source, because these test results may indicate either ASB or UTI.
Treatment and Prevention of Asymptomatic Bacteriuria
In discussing treatment of any disorder, two general questions should be considered:
1. Does the benefit of treatment outweigh the risk?
2. Does treatment improve morbidity and mortality?
The risk-benefit analysis for antibiotic prescriptions in the elderly can easily tilt to the side of risk. Renal clearance of medications declines with age,2 making the elderly at increased risk for adverse drug reactions. Decreased clearance also enhances the possibility of a drug-drug interaction, which is particularly important in the elderly who are often already taking several medications. Also, patients over the age of 50 frequently have a UTI from a resistant organism; 45% of elderly patients presenting to the emergency department with a UTI had an infection with a multidrug-resistant organism.1 While age was not specifically related to the probability of having an infection due to a resistant organism, repeated antibiotic use contributed. Some now suggest, therefore, that the fluoroquinolone class of antibiotics should be first-line therapy in the elderly, but these medications may lower seizure threshold and increase cardiac arrhythmias.17 Restricting the use of any medication in the geriatric population is advisable because of these reasons, and judicious use of antibiotics is always recommended to avoid induction of resistance. Before starting an antibiotic in an elderly patient, these risks need to be considered.
Antibiotic therapy is not necessary for ASB. True UTI does occur in patients with ASB,16,31 but the consensus is that treatment of ASB is not beneficial.2,5-7,9,17,23,32 The two studies suggesting that ASB leads to symptomatic infections were in women with diabetes. One found a correlation of ASB with UTI in persons with type 2 diabetes,31 and the other suggested that ASB was a “borderline” risk.16 In contrast to these, a placebo-controlled trial of women with diabetes did not demonstrate a difference in the overall rate of UTI development with and without treatment of ASB.32 Treatment of ASB does not reduce the occurrence of UTI. Also, chronic genitourinary symptoms, including incontinence, do not improve after treating ASB.5,7,9 The risks are greater than the benefit because morbidity is unchanged with treatment.
Not only is morbidity unaltered by treatment of ASB, but mortality is not increased by the presence of ASB. The trial that demonstrated no differences in the rate of UTI occurrence in women with diabetes also revealed no changes in renal function in those treated when compared to the placebo group.32 ASB does not promote renal function decline,5-7,9,32 and it has not been correlated with an increase in mortality among the elderly.5-7,9,33,34 In a group of elderly women, death rates were similar among those who were considered “ever positive” for bacteriuria and those who were “never positive.”34 Additionally, attempts to eliminate or cure ASB with antibiotic therapy fail because of recurrence.4,6,7,22,23 After treatment of nursing home residents, 50-70% did not have bacteriuria at 1 week, but just 4 weeks after treatment only 30-50% did not have bacteriuria.6 The answer to the original proposed questions in relation to treatment of ASB in the elderly, therefore, is that the risks are greater than the benefit, because morbidity and mortality are neither changed by the presence of ASB nor by treatment with antibiotics.
Treatment of ASB does temporarily eliminate bacteriuria, and this is useful in patients scheduled to undergo a urologic procedure that involves disruption of the mucosa and bleeding2,5,6; otherwise, antibiotics are not indicated.
If treatment of ASB is not necessary, then screening and prevention are also unnecessary. Good toileting behaviors and perineal hygiene should not be discouraged, but a strict regimen is not necessary for attempting to prevent ASB. The U.S. Preventive Services Task Force and the Infectious Diseases Society of America do not recommend screening for ASB in the elderly.5,35 Also, there is not adequate evidence to recommend routine use of ERT or cranberry juice for the prevention of ASB.6,11,17
Importance of Recognizing Asymptomatic Bacteriuria
Treatment of ASB promotes the development of resistant bacteria. Thirteen percent of the U.S. population is now 65 years or older;17 it is projected that by the year 2050, 20% will be over age 65.3 If these population growth projections hold true, then over the next 45 years, one-fifth of the U.S. population may erroneously be considered to have UTI as their most common infectious disease. Consequently, antibiotic prescriptions for the urinary tract will increase; there is a looming crisis.3 Recognizing ASB as a subset of these patients and not treating it will help to curtail this potential crisis. Treating ASB temporarily eliminates the bacteria, but recurrence happens with resistant organisms.4,6 Thirty to 60% of antibiotics that are prescribed in long-term care facilities for UTI are actually inadvertent and unnecessary treatment of ASB.1 Avoiding treatment of ASB will aid in preserving the longevity of antibiotics commonly prescribed for the urinary tract.
ASB does not lead to further morbidity or mortality in the elderly, and, therefore, treatment is not warranted on a routine basis. ASB and UTI appear similar on urinalysis, which entices clinicians to initiate antibiotic therapy, but they need to be recognized as separate entities and distinguished by the presence or absence of specific urinary tract symptoms. The geriatric population is increasing, and ASB will be encountered more often. Treating ASB will promote the development of resistant organisms and further complicate the care of geriatric patients.
The authors would like to thank Dayna Owen White, MA, English Literature, for editorial suggestions and assistance in preparing this manuscript. The authors report no relevant financial relationships. Dr. Benton is Assistant Professor and Associate Residency Program Director; Dr. Nixon-Lewis is Assistant Professor and Residency Program Director, Department of Family and Community Medicine, Texas Tech University Health Sciences Center at Amarillo School of Medicine.
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