When Laboratory Confirmation Fails Clinical Reality

Babesia clinical diagnosis may be necessary when laboratory tests fail to confirm infection. Investigators from ISDH and the Centers for Disease Control and Prevention (CDC) examined specimens from 14 patients diagnosed with Lyme disease and B. microti. They tested for Babesia infection by Giemsa-stained blood smears, PCR (polymerase chain reaction), and indirect fluorescent antibody (IFA) for total immunoglobulin to B. microti.

The only clinical manifestation consistent with Lyme disease included unspecified rashes in 3 patients. Anemia and fever, symptoms associated with Babesia, were identified in 2 patients.

None Met CDC Surveillance Criteria

As a result, investigators concluded that none of the patients “fulfill the national surveillance case definition for non–transfusion-associated babesiosis,” writes Brown from ISDH.

Two of the 12 patients had a B. microti IFA titer of 1:64 but this is considered “insufficient evidence” by the CDC for a Babesia diagnosis. All other testing was negative. None of the patients who initially tested positive for B. microti serology were confirmed as positive.

The CDC and ISDH concluded that their “laboratory-based investigation does not suggest a cluster of Lyme disease or babesiosis cases among these patients.”

This conclusion creates a troubling paradox: fourteen patients were clinically diagnosed with Babesia by treating physicians. Laboratory investigation found no confirmation. Does this mean the diagnoses were wrong, or does it mean the laboratory investigation was inadequately designed to detect Babesia in these specific patients?

Why Babesia Tests Can Be Negative

There are several possible reasons investigators could not find laboratory confirmation of B. microti.

First, they looked at lab results long after the onset of symptoms, when Giemsa-stained blood smears and PCR would typically be negative.

“Patient specimens were collected a median of 172 days (range = 22–348 days) after reported illness onset date,” writes Brown.

This timing is critical. PCR detects Babesia DNA circulating in blood. During acute infection, parasitemia is high and PCR sensitivity is greatest. As infection progresses and immune response develops, parasitemia declines. By 172 days after symptom onset—nearly 6 months—most patients with treated Babesia will have cleared detectable parasitemia.

Blood smears visualize parasites inside red blood cells. Like PCR, sensitivity depends on parasite density. Low-level chronic infection or treated infection produces parasitemia too low for microscopic detection.

Treatment Effects on Laboratory Results

Secondly, investigators looked at serologic specimens after patients had already received on average 3 antimicrobial agents (range = 1–6).

Treatment and a delay in testing may have altered the laboratory results.

Antimicrobial therapy reduces parasite burden. If patients received atovaquone and azithromycin—the standard Babesia treatment—parasitemia would decline or clear entirely. PCR and blood smear performed after treatment would show negative results even when infection was genuine.

The serologic testing (IFA antibodies) should theoretically remain positive after treatment, as antibodies persist. However, two patients did show IFA titers of 1:64. The CDC considers this “insufficient evidence” for diagnosis, requiring titers of 1:256 or higher for surveillance case definition.

This creates a catch-22: test too early (before antibodies develop), serology is negative. Test during acute illness, parasitemia is high but patients may not have accessed testing. Test after treatment, parasitemia has cleared and antibody titers may not meet arbitrary thresholds.

Limitations of the Babesia Investigation

Investigators did not fully examine the factors that led the doctor to make a clinical diagnosis of Babesia. “ISDH did not conduct patient interviews or chart reviews; demographic and clinical data were obtained from the CDC specimen submission form,” states Brown.

This is perhaps the most significant limitation. The treating physicians diagnosed Babesia based on clinical presentation, exposure history, symptoms, and likely response to treatment. None of this clinical context was evaluated.

Without patient interviews, investigators couldn’t determine: What symptoms led to Babesia diagnosis? Was there documented tick exposure? Did symptoms include night sweats, air hunger, or other Babesia-specific manifestations? Did symptoms improve with Babesia-directed treatment? Were laboratory findings consistent with Babesia (anemia, thrombocytopenia, elevated liver enzymes)?

Chart reviews would have revealed this clinical reasoning. Instead, investigators relied solely on laboratory testing performed months after diagnosis and treatment—testing specifically designed for surveillance purposes rather than clinical diagnosis.

When Babesia Clinical Diagnosis Is Appropriate

Doctors are advised to make a clinical diagnosis of Babesia, Brown concludes. “Lyme disease and babesiosis should be considered in the differential diagnosis for patients with clinically compatible illness and potential exposure to I. scapularis ticks in areas where the pathogens are present.”

This case series underscores the importance of not relying solely on laboratory confirmation when tick-borne co-infections are suspected — particularly when testing is delayed or patients have already received treatment.

The authors’ conclusion validates clinical diagnosis despite their inability to confirm cases through laboratory testing. This represents an important acknowledgment: Babesia diagnosis depends on clinical judgment, not solely on laboratory results that may be negative for technical reasons unrelated to actual infection status.

CDC Surveillance Criteria vs Clinical Diagnosis

The CDC surveillance case definition for babesiosis requires specific laboratory confirmation: parasites visualized on blood smear, positive PCR, or IFA antibody titer ≥1:256.

These criteria were designed for public health surveillance—tracking disease incidence and geographic distribution. They were not designed as clinical diagnostic criteria for individual patient care.

The distinction matters. Surveillance definitions intentionally favor specificity over sensitivity to avoid false-positive case reports inflating disease statistics. This means some genuine cases will be missed by surveillance criteria.

Clinical diagnosis requires different standards. When patient presents with compatible symptoms, tick exposure in endemic area, and laboratory findings suggesting Babesia (even if not meeting surveillance thresholds), clinical diagnosis is appropriate—particularly when treatment produces symptom improvement.

Frequently Asked Questions

Can you have Babesia with negative laboratory tests?
Yes. This study examined 14 patients clinically diagnosed with Babesia—none met CDC surveillance criteria due to negative or insufficient laboratory results. Testing performed median 172 days after symptoms, after average 3 antimicrobials, will often be negative even when infection was genuine.

Why would Babesia tests be negative after treatment?
PCR and blood smears detect parasites in blood. After treatment, parasite burden declines or clears, producing negative results. Antibody tests should remain positive, but titers may fall below CDC thresholds (1:256) while still indicating past infection (1:64).

When should doctors make a clinical Babesia diagnosis?
When patients have clinically compatible illness (fever, night sweats, anemia, air hunger) and potential exposure to Ixodes ticks in endemic areas. The authors conclude clinical diagnosis is appropriate even when laboratory confirmation is unavailable or negative.

What’s the difference between CDC surveillance criteria and clinical diagnosis?
CDC surveillance criteria require specific laboratory thresholds for public health tracking. Clinical diagnosis considers full clinical picture: symptoms, exposure history, laboratory findings, and treatment response—even when surveillance criteria aren’t met.

Should Babesia treatment wait for positive lab results?
Not necessarily. When clinical suspicion is high based on symptoms and exposure, empiric treatment may be appropriate. Waiting for laboratory confirmation—particularly when testing is delayed or patients already received antibiotics—may prolong illness unnecessarily.

Clinical Takeaway

This investigation demonstrates the fundamental tension between laboratory surveillance requirements and clinical diagnosis of Babesia. Fourteen patients were clinically diagnosed with B. microti infection by treating physicians. CDC and ISDH investigators tested specimens from these patients using Giemsa-stained blood smears, PCR, and IFA antibody testing. None met CDC surveillance case definition for babesiosis. Investigators concluded their laboratory investigation “does not suggest a cluster of babesiosis cases among these patients.” This conclusion dismisses fourteen clinical diagnoses based solely on laboratory testing performed under conditions specifically designed to produce negative results. The timing was catastrophically wrong. Patient specimens were collected median 172 days after illness onset—nearly six months later. At this timepoint, PCR and blood smears would typically be negative even in genuine cases. Parasitemia peaks during acute illness, then declines as immune response develops and treatment proceeds. By six months post-symptom onset, detectable parasites have usually cleared. Additionally, patients had already received average three antimicrobial agents before testing. Antimicrobial therapy reduces parasite burden. If patients received atovaquone and azithromycin—standard Babesia treatment—parasitemia would decline or clear entirely, producing negative PCR and blood smears even when initial infection was genuine. Two patients did show IFA titers of 1:64, indicating antibody response to B. microti. However, CDC surveillance criteria require titers ≥1:256, so these were deemed “insufficient evidence.” The arbitrary threshold dismisses serologic evidence of past infection that doesn’t meet surveillance cutoffs designed for public health tracking rather than clinical diagnosis. The most significant limitation: investigators never examined the clinical reasoning behind original diagnoses. No patient interviews. No chart reviews. Demographic and clinical data obtained only from CDC specimen submission forms. The treating physicians diagnosed Babesia based on clinical presentation, exposure history, symptoms, laboratory findings, and treatment response. None of this clinical context was evaluated. Without it, investigators assessed only whether laboratory testing performed six months post-diagnosis met surveillance criteria—not whether patients actually had Babesia. The authors’ own conclusion validates this interpretation: “Lyme disease and babesiosis should be considered in the differential diagnosis for patients with clinically compatible illness and potential exposure to I. scapularis ticks in areas where the pathogens are present.” This endorses clinical diagnosis despite inability to confirm cases through delayed laboratory testing. The lesson for clinicians: Babesia remains a clinical diagnosis for many patients. Laboratory confirmation supports diagnosis when positive, but negative results don’t rule out infection—particularly when testing is delayed, patients have received treatment, or results fall just below arbitrary surveillance thresholds. Clinical judgment considering full clinical picture—symptoms, exposure, laboratory findings, treatment response—guides diagnosis more reliably than laboratory surveillance criteria applied months after treatment.