NHSN CLABSI: Pneumonia… Or Is It?
The Question
A patient with a central line develops fever, a new right lower-lobe consolidation, Staphylococcus aureus in a respiratory culture, and Staphylococcus aureus bacteremia.
The medical team diagnoses pneumonia.
Does the patient meet NHSN PNEU criteria, and can the bloodstream infection be attributed to pneumonia?
The Case
Hospital Day 1
A 69-year-old man is admitted to the ICU following cardiac arrest.
A PICC line is placed and accessed.
Hospital Day 4
The patient develops a fever of 38.7°C.
Chest imaging shows a new right lower-lobe consolidation.
Blood cultures, a urine culture, and an expectorated sputum culture are collected.
The urine culture shows no growth.
Blood cultures grow: Staphylococcus aureus.
The sputum Gram stain shows:
few WBCs
few epithelial cells
mixed flora
The sputum culture grows: Moderate growth respiratory flora with moderate growth S. aureus.
The clinical team documents pneumonia with S. aureus bacteremia thought to be secondary to the pneumonia.
Hospital Day 5
Repeat chest imaging shows marked improvement of the right lower-lobe opacity, with the finding now favored to represent atelectatic change rather than pneumonia.
No dyspnea, tachypnea, new or worsening cough, rales, bronchial breath sounds, worsening gas exchange, or increased respiratory secretions are documented.
What Do You Think?
The patient has:
fever
a new right lower-lobe consolidation
S. aureus recovered from sputum
S. aureus recovered from blood
a physician diagnosis of pneumonia
an eligible PICC
Does this bloodstream infection meet NHSN criteria for a CLABSI?
Yes
No — the bloodstream infection is secondary to pneumonia
Not enough information
Reveal the Answer
Answer: Yes.
Despite the clinical diagnosis of pneumonia, the patient does not meet an NHSN PNEU definition. Therefore, the S. aureus bloodstream infection cannot be attributed to pneumonia for NHSN surveillance.
The patient meets LCBI 1, and because the PICC had been in place for more than 2 consecutive calendar days and was present on the date of event, the event is reported as a CLABSI, assuming no other NHSN-defined secondary source is identified. NHSN requires a site-specific infection definition to be fully met before a BSI can be attributed to that site.
Now let’s walk through the traps.
Step 1: Start With the Imaging
The patient has a new right lower-lobe consolidation.
Seems easy enough, right?
Not quite.
Our patient was admitted following cardiac arrest and has underlying cardiac disease. Certain underlying cardiac and pulmonary conditions can mimic pneumonia on chest imaging. For patients with underlying cardiac or pulmonary disease, NHSN requires two or more serial chest imaging results demonstrating eligible findings that are new and persistent or progressive and persistent.
And persistence matters for everybody.
NHSN recommends reviewing subsequent imaging over several days when available. If later imaging shows rapid resolution or attributes the finding to another condition, persistence has not been demonstrated.
That is exactly what happened here.
Hospital Day 4: New right lower-lobe airspace opacity/consolidation concerning for pneumonia versus atelectasis
Hospital Day 5: Marked improvement in right basilar opacity, most consistent with resolving atelectasis.
The imaging requirement is not met.
And once one required portion of the PNEU definition fails, we cannot simply skip over it because the rest of the case “looks like pneumonia.” We could stop here, but let's continue.
Step 2: Does the Patient Have a Qualifying Sign or Symptom?
The fever counts toward the first portion of the PNU2 adult sign-and-symptom criteria.
But fever alone is not enough.
For PNU2, an adult patient must also have at least one qualifying respiratory finding, such as:
new-onset purulent sputum or a change in character of sputum
increased respiratory secretions or suctioning requirements
dyspnea, tachypnea, or new/worsening cough
rales or bronchial breath sounds
worsening gas exchange
Our patient has none of those documented.
“But wait. We sent sputum. Wasn’t it purulent?”
That brings us to trap number two.
Step 3: Does the Patient Have New-Onset Purulent Sputum or a Change in Sputum Character?
This is an easy place to make an assumption.
The patient produced sputum, the specimen was sent to the laboratory, and S. aureus grew.
But that does not automatically mean the patient had new-onset purulent sputum.
For NHSN surveillance, purulent sputum has a specific laboratory definition. The respiratory secretions must contain:
≥25 neutrophils AND ≤10 squamous epithelial cells per low-power field (x100).
NHSN requires laboratory confirmation because written clinical descriptions of purulence can be highly variable. Words such as purulent, thick, yellow, or green alone do not establish this criterion.
Our laboratory reports the direct examination semi-quantitatively rather than providing an exact number of cells per low-power field.
The Gram stain in this case was reported as:
Few WBCs
Few epithelial cells
Mixed flora
Semi-quantitative reporting simply means the laboratory uses descriptive categories, such as rare, few, moderate, or many, instead of reporting an exact microscopic cell count.
When a laboratory uses this type of reporting, NHSN provides guidance for translating those terms into the purulent-sputum criterion. If the laboratory has its own defined correlation between its semi-quantitative terminology and the actual number of cells per low-power field, that correlation should be used.
In our case, “few WBCs” does not establish the neutrophil burden required for NHSN-defined purulent respiratory secretions.
But do not stop here.
New-onset purulent sputum is only one way to meet this respiratory criterion.
NHSN also allows:
a change in character of sputum
increased respiratory secretions
increased suctioning requirements
A documented change in sputum color, consistency, odor, or quantity can therefore satisfy this portion of the definition even when the specimen does not meet the microscopic definition of purulent sputum.
In this case, however, there is no documentation of a change in sputum character, increased respiratory secretions, or increased suctioning requirements.
So this respiratory criterion is not met.
And that distinction matters: the Gram stain tells us whether the secretions meet NHSN’s definition of purulence; the medical record tells us whether the character of the sputum changed. We need to evaluate both.
Step 4: But the Sputum Grew the Same Organism as the Blood
This is probably the biggest temptation in the case.
Sputum: S. aureus
Blood: S. aureus
Same bug.
Surely that makes the bloodstream infection secondary to pneumonia?
No.
This was expectorated sputum from a non-ventilated patient.
For the PNU2 laboratory criterion involving a minimally contaminated lower-respiratory specimen, NHSN specifically identifies specimens such as:
bronchoalveolar lavage
protected specimen brushing
endotracheal aspirate
The 2026 PNU2 algorithm requires a qualifying quantitative or corresponding semiquantitative result from an eligible minimally contaminated lower-respiratory specimen.
An expectorated sputum specimen from a non-ventilated patient is not one of those minimally contaminated specimens.
The fact that S. aureus grew does not change the specimen type.
Step 5: What Does “Moderate Growth” Mean?
This is where microbiology terminology creates another trap.
Clinical microbiology laboratories commonly report respiratory culture growth semi-quantitatively, using terms such as:
rare → light → moderate → heavy
or an equivalent numerical scale:
1+ → 2+ → 3+ → 4+
These terms describe the relative amount of organism recovered on the culture media rather than providing an exact CFU/mL result.
For NHSN PNU2, a semi-quantitative result can correspond to the required quantitative threshold. In the absence of a laboratory-specific correlation, NHSN considers moderate, heavy, many, or numerous
growth—or 2+, 3+, or 4+ growth—to meet the corresponding threshold.
So our moderate growth of S. aureus is not the problem.
The specimen is.
For this PNU2 laboratory criterion, the organism must be recovered at the qualifying amount from an eligible minimally contaminated lower-respiratory specimen.
Three things matter: the specimen, the organism, and the amount of growth.
Our result was:
Expectorated sputum — moderate growth S. aureus.
The amount of growth is potentially qualifying.
S. aureus is an eligible pathogen.
But this was expectorated sputum from a non-ventilated patient, which NHSN does not consider a minimally contaminated lower-respiratory specimen for this criterion.
Therefore, this respiratory culture cannot be used to satisfy the PNU2 minimally contaminated respiratory-specimen criterion, even though the organism and amount of growth would otherwise be acceptable.
Step 6: But Could the Blood Culture Itself Be Used for PNU2?
Now we get to the sneaky part.
S. aureus identified from blood is an eligible PNU2 laboratory element.
So the fact that the expectorated sputum culture does not qualify does not, by itself, automatically rule out PNU2. The blood specimen can potentially satisfy the PNU2 laboratory component.
But PNU2 requires all three pieces:
Imaging + Signs/Symptoms + Laboratory evidence
within the appropriate NHSN timeframe.
Our patient still fails the definition because:
the imaging does not demonstrate the required persistence, and
no qualifying respiratory sign or symptom is present.
So the blood culture cannot rescue an otherwise incomplete PNU2 definition.
This is why we walk the entire definition.
Step 7: Does the Patient Meet PNU1 Instead?
No.
PNU1 does not require laboratory evidence, but it still requires qualifying imaging and signs/symptoms.
Those requirements are not met here.
And there is another important point:
Even if the patient had met PNU1, we still could not assign the S. aureus bacteremia as secondary to a PNU1 event. NHSN states that pathogens are not reported for PNU1 because PNU1 has no laboratory element. Therefore, a secondary BSI cannot be assigned to PNU1.
Secondary BSI attribution to PNEU requires PNU2 or PNU3 plus one of the NHSN secondary-BSI scenarios.
So What About the S. aureus in the Blood?
Only now do we return to the bloodstream infection.
For NHSN, you cannot simply document: “Bacteremia secondary to pneumonia.”
You first have to prove that an NHSN-defined infection exists at the proposed primary site.
NHSN instructs facilities to first fully meet the applicable site-specific definition and then apply the Secondary BSI Guide. If no qualifying site-specific infection is identified and the BSI otherwise meets LCBI criteria, it remains a primary bloodstream infection.
Our patient has S. aureus identified from blood.
S. aureus is a recognized pathogen rather than an NHSN common commensal, so its identification from one or more blood specimens can meet the microbiologic requirement for LCBI 1, provided the organism is not related to an infection at another NHSN-defined site.
We looked.
The pneumonia does not meet NHSN criteria.
The urine culture is negative.
No other qualifying source has been identified.
The BSI therefore remains primary.
And because the PICC line was placed and accessed on Hospital Day 1 and the BSI occurred on Hospital Day 4, the line meets the central-line timing requirement.
Final NHSN determination: CLABSI.
The Trap
This patient looks like pneumonia.
The physician diagnosed pneumonia.
The imaging initially looks like pneumonia.
The sputum grows S. aureus.
The blood grows S. aureus.
Clinically, attributing the bacteremia to the lungs may seem entirely reasonable.
But NHSN surveillance is not asking:
“What does the clinical team think happened?”
It is asking:
“Does the patient meet the NHSN surveillance definition?”
Those are not always the same answer.
So Where Does That Leave Us?
The patient had:
an eligible PICC line
S. aureus bacteremia meeting the microbiologic component of LCBI 1
an initial chest image concerning for pneumonia
subsequent imaging that did not demonstrate persistence
no qualifying NHSN respiratory sign or symptom
expectorated sputum that was not a qualifying minimally contaminated LRT specimen
moderate growth S. aureus, an amount that could meet the semiquantitative threshold if the specimen itself were eligible
no NHSN-defined pneumonia to which the BSI could be attributed
The event is a CLABSI.
And perhaps the most important lesson from this case:
Looking for another source is absolutely part of the investigation.
But for NHSN surveillance, a suspected source only counts if it meets the applicable site-specific definition.
That is the question that determines whether the BSI can be attributed elsewhere.
References and Resources
