Diphtheroids in a urine test result refer to a group of bacteria, primarily members of the genus Corynebacterium, that are not the species causing the severe disease diphtheria. These bacteria are common residents of the human body and are frequently encountered in clinical laboratory settings. In many cases, it represents a harmless event rather than a true infection. Understanding the bacteria’s natural habitat and the method of sample collection determines if the result is a non-issue or requires medical attention.
Identifying Diphtheroids and Their Natural Habitat
Diphtheroids are Gram-positive rods, meaning they retain the crystal violet stain used in laboratory tests. They often display a characteristic club-shaped or pleomorphic morphology under a microscope. The diphtheroid species found in urine are non-toxin-producing, unlike Corynebacterium diphtheriae. They are typically aerobic and non-spore-forming.
These organisms are an established part of the normal human microflora, colonizing the skin, mucosal membranes, and external genitalia. Their presence around the urethral opening is relevant to urine sample analysis. Because they are widespread on the body surface, these bacteria frequently find their way into a collected sample.
Interpreting Diphtheroids in Urine Samples
The most common reason diphtheroids appear in a urine culture is external contamination. When a patient provides a “clean-catch” midstream urine sample, bacteria from the surrounding skin, including the periurethral area, can inadvertently be flushed into the collection cup.
Laboratory staff often treat a low colony count of diphtheroids as a non-significant result, particularly if the patient has no symptoms of a urinary tract infection. A count below 50,000 colony-forming units per milliliter (CFU/mL) in an otherwise healthy, asymptomatic patient is usually dismissed as contamination.
The clinical laboratory may report the finding as “mixed flora” or “skin contaminant.” They often do not proceed to full species identification or antibiotic susceptibility testing. This practice reflects the understanding that the bacteria did not originate from the bladder or kidneys. If the initial sample collection was poorly executed, the presence of these bacteria indicates a need for a re-test using a more sterile collection method.
When Diphtheroids Signal a True Infection
While most findings are benign, diphtheroids can signal a true infection in specific clinical scenarios. The presence of a high colony count, typically defined as greater than 100,000 CFU/mL, indicates that the bacteria are replicating within the urinary tract.
A true infection is more likely when the sample is collected using a sterile technique, such as a catheter insertion or suprapubic aspirate, where external contamination is minimized. In these cases, even a lower count of diphtheroids is often considered significant. The patient’s clinical status also plays a role in interpreting the result.
Patients who are immunocompromised, have indwelling urinary catheters, or have prosthetic devices are at higher risk for infection. Specific species, such as Corynebacterium urealyticum or Corynebacterium striatum, are recognized uropathogens capable of causing serious urinary tract infections, especially in these vulnerable populations. When these findings coincide with symptoms like painful urination, urinary frequency, or flank pain, the diphtheroid is considered the causative agent.
Medical Follow-Up and Treatment Decisions
The decision to pursue medical follow-up or treatment for diphtheroids in urine depends on integrating the laboratory result with the patient’s health history. If the patient is asymptomatic, medical guidelines advise against treating the finding, as this represents asymptomatic bacteriuria. Unnecessary antibiotic use contributes to antimicrobial resistance without providing clinical benefit to the patient.
If a true infection is suspected due to symptoms, a high colony count, or patient risk factors, the organism is fully identified and susceptibility testing performed. Many pathogenic diphtheroid species exhibit resistance to common antibiotics, including beta-lactams and fluoroquinolones. This often necessitates the use of agents like vancomycin or linezolid. The healthcare provider uses the susceptibility data to select the most effective antibiotic.