A culture and susceptibility report is really 2 documents stapled together: what grew, and which drugs stop it growing. The standard turn is 48 hours, roughly 24 for the organism and 24 more for the susceptibility read. Whether those 48 hours mean anything is decided before the sample ever reaches the lab, by 3 sampling rules.
Read the report in this order: site, organism, colony count, then the 3 columns of 敏, 中, 耐 (susceptible, intermediate, resistant). A name at the top with no count and no site is a rumour, not a result.
Rule 1: sample before the first dose
Antibiotics shrink the viable count within 24 to 48 hours. A cat that has had 3 days of anything can culture negative and still carry the infection, and a negative report on a treated cat tells you almost nothing. Common practice is to sample before the first dose, or 48 to 72 hours after the last one; long-acting injections need longer, and the exact interval is the treating veterinarian's call.
Note the date and hour of the last dose on the lab form; a lab that knows the drug and the interval can pick the right medium.
Rule 2: the tissue, not the surface
Swabs of surfaces grow the surface. A skin swab taken over intact hair coat returns 2 to 4 organisms that live there and say nothing about the wound underneath. The samples that answer a question:
- Urine: cystocentesis, 3 to 5 mL, not a free-catch sample off a litter tray.
- Wound or abscess: aspirate with a needle, or a tissue piece from the edge; a surface swab is the last choice.
- Ear: swab of the horizontal canal under sedation, after the worst of the debris is removed.
- Nasal or sinus: deep swab, both nostrils, separate containers.
Rule 3: volume, container, temperature
Urine needs at least 1 mL, refrigerated at 4 °C, and delivered within 6 hours if possible and 24 hours at the outside. Past 24 hours at room temperature the container grows a mixed flora that looks like a urinary infection and is not one. Swabs go into Amies transport medium at room temperature and are stable for 24 to 48 hours. Tissue goes into saline, never formalin, because formalin fixes the bacteria the lab needs to grow.
Label the exact site. Left ear and right ear are different specimens; a report that says ear without a side cannot be compared with the next one.
The 48 hours, hour by hour
Hour 0 the sample is plated on blood agar and a selective medium. By hour 18 to 24 there are visible colonies and a Gram stain: cocci in pairs, rods, or a mix. Hour 24 brings a subculture and an identification, by biochemical panel or mass spectrometry. Hour 40 to 48 the susceptibility panel is read, either as zones around disks or as a minimum inhibitory concentration in µg/mL. Slow growers such as Nocardia need 72 hours or more, and fungal cultures run 3 to 4 weeks.
Many labs phone a preliminary result at 24 hours: the Gram stain plus the colony count. That call is often enough to change which drug class is chosen, and it costs nothing to ask for it.
What 敏, 中 and 耐 actually mean
The 3 letters are 1 comparison: the drug's minimum inhibitory concentration against a published breakpoint. 敏 (susceptible) means the MIC sits at or below the breakpoint, so a standard exposure at that site should inhibit the organism. 中 (intermediate) means the MIC is close to the breakpoint: the drug may work where it concentrates, and may fail where it does not. 耐 (resistant) means the MIC is above the breakpoint, and no realistic exposure is expected to work.
Breakpoints are per drug, per organism and per body site, and the tables get revised. A drug judged 敏 in an old report can be 耐 in a new one for the same organism, because the breakpoint moved, not because the cat changed. Urine is the clearest case: a drug can read 中 on a blood breakpoint and still be useful in urine, where it concentrates many times over.
The specimen table
| Specimen | Minimum volume | Transport | Result time |
|---|---|---|---|
| Urine, cystocentesis | 1 to 3 mL | 4 °C, under 24 hours | 48 hours |
| Wound aspirate | 0.5 to 2 mL | room temp, under 24 hours | 48 to 72 hours |
| Ear swab | 1 swab, both ears 2 tubes | Amies, under 48 hours | 48 hours |
| Tissue biopsy | 0.5 cm cube | saline, under 24 hours | 48 to 72 hours |
| Blood culture | 1 to 3 mL per bottle | 37 °C, never refrigerated | 5 days |
| Fungal culture | 1 swab or 1 plate | room temp, under 48 hours | 21 to 28 days |
Six rows, and the last column is why owners get surprised: a fungal culture can take 4 weeks, and no amount of phoning moves it.
Mixed growth, and the count that matters
Three or more organisms with no clear dominant one usually means contamination, not a 3-organism infection. The colony count carries the interpretation: urine at 10³ CFU/mL from a cystocentesis is significant, the same number from a free-catch sample is not. Ask for the count in CFU/mL, in writing, and ask whether the lab considers the growth pure.
No growth, sick cat: 5 reasons
Antibiotics in the last 48 hours, a fastidious organism that needs a special medium (Mycoplasma and Ureaplasma both do), a sample under 1 mL, a delay over 24 hours, or a disease that is not bacterial at all. Any of the 5 makes a negative culture a question, not an answer, and the next step is a repeat with the right container.
Why the choice is not yours to make
The panel lists 8 to 12 drugs with 3 possible letters each. Turning 1 敏 into a plan demands the site (an abscess wall, the prostate, the eye and the bladder all behave differently), the cat's kidney and liver values, the weight to 0.1 kg, and every other drug on board. Sensitivity to a drug in a dish is not the same as success in a cat.
What you can do is ask 2 questions: which of these 敏 results has the narrowest spectrum, and does the report match the site we sampled. Narrower is not weaker; it is the choice that leaves the next infection treatable.
5 steps that make the 48 hours count
- Ask for the sample before the first dose, and write the last dose time on the form.
- Insist on the right specimen: cystocentesis for urine, aspirate or tissue for a wound.
- Deliver the same day; refrigerate urine at 4 °C, never freeze a swab.
- Request the colony count in CFU/mL and the actual MIC numbers, not just the 3 letters.
- Ask for the 24-hour preliminary call, so the plan can move a day earlier.
A white cell count taken at the same visit helps frame the report: the ranges and the stress response are explained in the WBC guide, and breed predispositions to skin and urinary infections sit in the breed quiz.
Frequently asked
The report says susceptible. Why does my vet not just use that drug?
Because susceptibility in a dish is 1 input, not the plan. The drug has to reach the site at a usable concentration, and an abscess wall, the prostate and the eye all behave differently from blood. Kidney and liver values, weight, and interactions also change what can be given, so the choice stays with the clinician.
Is a negative culture good news?
Only if the sample was taken correctly and the cat had no antibiotics in the previous 48 hours. Otherwise it means the organism was not recovered, not that it is absent. Common reasons are a fastidious species needing special media, a sample under 1 mL, or a delay of more than 24 hours in transport.
Why does the same organism come back 耐 for a drug that worked before?
Breakpoint tables get revised, and the same MIC can cross a new threshold without the organism changing at all. Resistance can also be real, selected by earlier courses. Either way the letter alone does not decide: ask the lab for the actual MIC number and compare it with the breakpoint used on that report.