How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Dr. Tharun Your Own Question
Dr. Tharun
Dr. Tharun, Urology Resident
Category: Urology
Satisfied Customers: 3682
Experience:  MBBS, DNB surgery and presently working as Urology resident towards the degree of Urologist.
Type Your Urology Question Here...
Dr. Tharun is online now
A new question is answered every 9 seconds

If I am taking Nitrofurantoin as a prophylactic against UTIs

This answer was rated:

If I am taking Nitrofurantoin as a prophylactic against UTI’s (100 mg/day for about six weeks now), how do I know whether or not I have a UTI? The way I usually knew was very frequent urination (every ninety seconds; that cannot occur when using a catheter; sometimes after/while using catheters I feel like I need to urinate but cannot of course). So just wondering how I know if I would have a UTI.

Further, if I went into the doctor’s office/after lab office, how would they diagnose whether I had a UTI? I take it test strips are not effective when on Nitrofurantoin? Or is it more accurate that if an infection the bacteria will still show up because the Nitrofurantoin is not killing them? And if the strips do not work, would a culture show whether a UTI exists?

Thank you.

The urine strips are ineffective when on antibiotic because the all the bacteria are killed by the antibiotic and hence the urine becomes sterile.So that is aim of any antibiotic - to make the urine sterile.Hence the culture will also come negative.

The main criteria to decide whether you have UTI or not is the symptom complex.If you are symptom free you are very less likely to have an UTI.So when on antibiotic if you develop symptoms then you have an UTI and it will detected on various tests.

It is symptoms which governs the whole thing.So many old patients have positive urine tests without symptoms .They are never treated for infection.As rightly said treat the patient not the investigations.

I hope this answers your question.


Customer: replied 4 years ago.

Don't follow most of your response.

If all the bacteria are killed, then how can you a UTI?


And if can still get a UTI, then how do you know as a patient that you have a UTI? What are the symptoms?


How would a doctor diagnose whether you had a UTI if taking antibiotics?


And if as you say many older patients have UTI without symptoms, isn't that dangerous? If so. what do you recommend to avoid this?


Thank You. I have follow-up questions that will be asked as a new question(s).

Hello Greg,

If all the bacteria are killed, UTI cannot occur.Only if the antibiotic is ineffective or bacteria virulent and resistant UTI can occur.UTI occurs due to reinfection by the same bacteria or another bacteria after stopping of antibiotic.

If you are on antibiotics and have an UTI, the symptoms will be same as any normal UTI.Frequency, Urgency, burning, lower abdominal pain , fever etc.

UTI can be diagnosed with a urine test and urine culture only.So if they are positive you have an UTI.

In older patients it is a norm to treat only symptomatic UTI.Older patients can have pus cells in the urine without infection.It is called pyuria of aged.So it is not necessary to treat every positive urine test in an elderly unless it is symptomatic.Sometimes bacteria can be normally residing in the urinary system without causing harm.They are called commensals.

I hope this clarifies your doubt.


Customer: replied 4 years ago.

Ok. Please clarify initial statement that culture will also come negative, and later apparenlty conflicting statement that UTI can be diagnosed with a urine test and urine culture only.

And if no bacteria means no UTI, then UTI must mean bacteria. So why wouldn't the strips work? But a culture would work (see first sentence above).


Thank you.



The culture will come negative if the urine is sterile - that is devoid of bacteria as they are killed by the antibiotic.That is why culture is always done before starting the antibiotic.Once antibiotic is started the culture is negative, so UTI is responding and being treated.

Strips works by detecting the pus cells which contain specific enzymes which cause chemical changes as indicated by change in color of the strip.So it mainly detects pus cells in the urine which is an indirect evidence of UTI.

Culture means actual growth of the bacteria in medium which is almost confirmatory that the bacteria were present in the urine.


Customer: replied 4 years ago.

I think you are missing my point.


I am taking Nitrofurantoin as a prophylactic against UTI’s (100 mg/day for about six weeks now), Can I still get a UTI? Only if the bacteria becomes resistant to the Nitrofurantoin? I have heard it is possible, but probably not after just six weeks. I have heard various opinions; my pharmacist says she knows of several men and women using Nitrofurantoin as a prophylactic for years without a problem; yet, one urologist said that taking Nitrofurantoin every day will result in resistance 100% of the time.


You state that the culture will come back negative if the urine is sterile - that is devoid of bacteria as they are killed by the bacteria. But if somehow the bacteria becomes resistant, you can still get a UTI. Would that not be detectable by a strip and/or culture since now bacteria are present?



Yes, culture and strip test will detect the bacteria and pus cells if they are resistant against the prescribed antibiotic.

You can get a UTI while on prophylactic antibiotics and it is called break through infection.And culture will detect the infection in that case too.

Dr. Tharun and other Urology Specialists are ready to help you
Customer: replied 4 years ago.

So the only way you can obtain a UTI if taking Nitrofurantoin is if the bacteria becomes resistant, or can you get a UTI even if the bacteria does not become resistant? One reason I ask if that an expert told me that for the amount of urine I retain, that 50 g per day would not be sufficient; e.g., bacteria greater than Nitrofurantoin can handle.


How likely is it to receive a UTI while taking Nitrofurantoin? What have you seen as far as resistant strains of bacteria evolving? Have you seen doctors alternate between Nitrofurantoin for a period of time than switching to another antibiotic for another period of time and then back again to prevent this from occurring? Your opinion/recommendation? Thank you.




you can get an UTI if the bacteria are resistant to the given antibiotic.Urine dwell time is one of the factors for re-infection.If urine dwell time is more , then naturally bacteria will get more time to divide as well as attack the urinary system.

Nitrofurontoin is almost lost in past as most of the doctors find it not very useful in prophylaxis and also its side effect in causing lung problems.

Most of the time, low dose cephalosporins are good enough for prophylaxis and provide a better shield against these bacteria.


Dr. Tharun and other Urology Specialists are ready to help you
Customer: replied 4 years ago.


I went to Urgent Care this afternoon after AZO test strips at home showed positive for leukocytes twice. The sample at urgent care showed low in leukocytes and the doctor prescribed 250g of Cipro twice a day for five days. Does that sound reasonable?


I have concern in that my symptoms were not overwhelming, and I don't know long how I may have had the UTI (that is, if the culture comes back positive). No blood was found in the urine.


I asked the doctor if after the 5 days it was safe to go back on the nitrofurantoin as a prophylactic, and he said that in the 5-day interim that may help defeat the resistance, and said we would cross our fingers that that would be the case. What is your view on that?


I finally have my appointment to see a new urologist on Thursday, May 16th. Perhaps I should ask her about cephalosporins as a prophylactic for UTI's. What is the cephalosporin usually prescribed. Doesn't that have the same concern for resistance? Should a patient rotate prophylactic antibiotics monthly to avoid this. I generally retain 300-400 ml of urine: I take it that makes it more likely for me to develop UTI's when using a catheter even with antibiotics?


I never had a UTI unless using a catheter. I am wondering whether in a risk/benefit analysis it is better for me not to use a catheter and risk any damage to my bladder from retention? I have had no kidney problems to date but worry that UTI's from catheter use could result in that. The only side effect to date is a thickening of the bladder wall.


I have asked quite a few questions here. Feel free to answer some, and I will ask the remainder in a new question to better compensate you for your time. Thank you.



Thank you.



Hi Greg,

Cipro seems like a reasonable choice.We cannot deny that.

If there is resistance to one antibiotic , it will not be resolved in 5 days.Once you break into infection even on antibiotic, it is better to stay away from the antibiotic for some time like months.

Cifixime can used for prophylaxis.

Bacteria are intelligent organisms and give them a small window , they will develop resistance.This will be never ending phenomenon with antibiotics and bacteria.

Change the antibiotics when it does not work, that is general protocol being followed in medical community.

Infections in the bladder will not cause damage to the kidneys.Only if the infection reaches the kidney which is rare, there can be damage.

Chronic retention of urine will eventually lead to kidney damage, that is sure thing.A sure thing triumphs over a rare chance.


Dr. Tharun and other Urology Specialists are ready to help you