Microbiology differs from hematology and clinical chemistry because most microbiological investigations do not have traditional “normal ranges.” Instead, microbiology relies on reference parameters that define the normal microbial flora, acceptable microbial loads, sterility standards, culture characteristics, antimicrobial susceptibility breakpoints, and molecular diagnostic thresholds. These reference values are essential for interpreting culture results, antimicrobial susceptibility testing, molecular assays, serological investigations, infection control surveillance, environmental microbiology, and clinical diagnostics.
Normal Human Microbiota
The human body harbors trillions of microorganisms that constitute the normal microbiome. Their distribution varies according to anatomical site.
Skin Microbiota
| Organism |
Normal Presence |
| Coagulase-negative Staphylococci |
Abundant |
| Cutibacterium acnes |
Abundant |
| Corynebacterium spp. |
Common |
| Micrococcus spp. |
Common |
Average Bacterial Density
| Site |
Microbial Density |
| Dry Skin |
10²–10⁴ CFU/cm² |
| Moist Areas |
10⁴–10⁶ CFU/cm² |
| Sebaceous Areas |
10⁵–10⁷ CFU/cm² |
Oral Microbiota
| Parameter |
Reference Value |
| Total Bacterial Count |
10⁸–10⁹ CFU/mL Saliva |
| Streptococcus spp. |
Dominant |
| Veillonella spp. |
Common |
| Actinomyces spp. |
Common |
| Neisseria spp. |
Common |
Gastrointestinal Microbiota
| Parameter |
Reference Value |
| Total Intestinal Bacteria |
10¹¹–10¹² CFU/g Stool |
| Anaerobe:Aerobe Ratio |
Approximately 1000:1 |
| Bacteroides spp. |
Dominant |
| Bifidobacterium spp. |
Common |
| Lactobacillus spp. |
Common |
| Enterococcus spp. |
Common |
Vaginal Microbiota
| Parameter |
Reference Value |
| Vaginal pH |
3.8–4.5 |
| Lactobacillus spp. |
Predominant |
| Nugent Score |
0–3 (Normal Flora) |
Sterility Standards for Clinical Specimens
Under normal physiological conditions, certain body sites are considered sterile.
| Specimen |
Normal Finding |
| Blood Culture |
No Growth |
| Cerebrospinal Fluid |
No Growth |
| Pleural Fluid |
No Growth |
| Peritoneal Fluid |
No Growth |
| Synovial Fluid |
No Growth |
| Bone Marrow Culture |
No Growth |
| Pericardial Fluid |
No Growth |
Blood Culture Parameters
Adult Blood Culture Volume
| Parameter |
Reference Value |
| Single Blood Culture Set |
20–30 mL |
| Number of Sets |
2–3 Sets |
| Collection Interval |
30–60 Minutes |
Urine Culture Reference Values
Significant Bacteriuria
| Colony Count |
Interpretation |
| <10³ CFU/mL |
Usually Contamination |
| 10³–10⁴ CFU/mL |
Possible Infection |
| ≥10⁵ CFU/mL |
Significant Bacteriuria |
Sputum Microscopy Parameters
Bartlett Score
| Parameter |
Reference Value |
| Acceptable Specimen |
>25 PMNs and <10 Epithelial Cells/LPF |
Cerebrospinal Fluid Microbiology
| Parameter |
Normal Value |
| Gram Stain |
No Organisms Seen |
| Bacterial Culture |
No Growth |
| Fungal Culture |
No Growth |
| Acid-Fast Bacilli |
Not Detected |
Antimicrobial Susceptibility Testing (AST)
Kirby–Bauer Method
| Parameter |
Reference Value |
| Mueller-Hinton Agar Depth |
4 mm |
| Inoculum Density |
0.5 McFarland |
| Bacterial Density |
1.5 × 10⁸ CFU/mL |
| Incubation Temperature |
35 ± 2°C |
| Incubation Time |
16–18 Hours |
McFarland Standards
| Standard |
Bacterial Density |
| 0.5 McFarland |
1.5 × 10⁸ CFU/mL |
| 1 McFarland |
3 × 10⁸ CFU/mL |
| 2 McFarland |
6 × 10⁸ CFU/mL |
Mycobacteriology Parameters
Acid-Fast Bacilli Smear Grading
| Grade |
Bacilli Observed |
| Scanty |
1–9 AFB/100 Fields |
| 1+ |
10–99 AFB/100 Fields |
| 2+ |
1–10 AFB/Field |
| 3+ |
>10 AFB/Field |
Parasitology Reference Parameters
Stool Examination
| Parameter |
Normal Finding |
| Protozoal Trophozoites |
Absent |
| Protozoal Cysts |
Absent |
| Helminth Eggs |
Absent |
| Helminth Larvae |
Absent |
Mycology Reference Parameters
Fungal Culture
| Parameter |
Reference Value |
| Blood Culture |
No Fungal Growth |
| CSF Culture |
No Fungal Growth |
| KOH Mount |
No Fungal Elements Seen |
Virology Reference Parameters
Viral Load Testing
| Parameter |
Reference Value |
| HIV Viral Load (Undetectable) |
<20–50 Copies/mL |
| HBV DNA |
Not Detected |
| HCV RNA |
Not Detected |
| CMV DNA |
Not Detected |
Molecular Microbiology Parameters
Real-Time PCR Performance
| Parameter |
Reference Value |
| PCR Efficiency |
90–110% |
| Correlation Coefficient (R²) |
≥0.99 |
| Ct Value (Strong Positive) |
<25 |
| Ct Value (Moderate Positive) |
25–30 |
| Ct Value (Weak Positive) |
30–35 |
| Ct Value (Borderline) |
>35 |
Infection Control and Environmental Microbiology
Operating Theater Air Quality
| Parameter |
Reference Value |
| Bacterial Load |
<10 CFU/m³ |
| Fungal Load |
<5 CFU/m³ |
Drinking Water Standards
| Parameter |
Reference Value |
| Total Coliforms |
0 CFU/100 mL |
| Fecal Coliforms |
0 CFU/100 mL |
| Escherichia coli |
0 CFU/100 mL |
Quality Control Parameters in Clinical Microbiology
| Parameter |
Reference Value |
| Incubator Temperature |
35–37°C |
| Refrigerator Temperature |
2–8°C |
| Freezer Temperature |
−20°C or Lower |
| Ultra-Low Freezer |
−70°C to −80°C |
| Autoclave Temperature |
121°C |
| Autoclave Pressure |
15 psi |
| Autoclave Exposure Time |
15–20 Minutes |
References
- Clinical and Laboratory Standards Institute. Performance Standards for Antimicrobial Susceptibility Testing (Latest Edition).
- World Health Organization.
- Manual of Clinical Microbiology.
- European Committee on Antimicrobial Susceptibility Testing.
- Mackie and McCartney Practical Medical Microbiology.
- Koneman's Color Atlas and Textbook of Diagnostic Microbiology.
- Bailey and Scott's Diagnostic Microbiology.
- Centers for Disease Control and Prevention.
- American Society for Microbiology.
- International Organization for Standardization.
