Case Study

Differential Diagnosis: Is it Bacterial Infection or Something Else?

Michael R. Broyles, PharmD – Director of Pharmacy and Lab Services

Age, sex:

67-year-old female

Chief complaint:

Generalized non-specific complaints of pain, new onset mild mental confusion. Patient was a poor historian.

Other complaints:

Pain in neck, shoulder, upper back, lower back, foot, and other multiple diffuse arthralgias

Significant medical history:

Recurrent urinary tract infections, hypertension, migraines, depression, generalized anxiety, fibromyalgia, osteoporosis

Admission labs, diagnostics:

Urinalysis took four attempts secondary to sludge in catheter: urine cloudy, nitrate positive, white blood cells/hpf = 5, bacteria = +4. White blood count = 9.6 x103/µL, PCT = 0.05 μg/L, BP 142/82 mmHg.

Admitting diagnosis:

Urinary tract infection

Treatment, interventions:

Admitted to floor. Started on ceftriaxone.

Day 1 at 8 am:

BP dropped to 90/58 mmHg, lactate = 2.7 mmol/L,

PCT = 0.05 μg/L.

New diagnosis:


Treatment, interventions:

Sepsis protocol initiated with fluid resuscitation. Ceftriaxone discontinued. Piperacillin/tzobactam and tobramycin started.

Day 1 at 5 pm:

BP remained at 98/60 mmHg, lactate = 4.3 mmol/L,

PCT = 0.05 μg/L, vancomycin started

Day 1 at 6 pm:

After reviewing the patient’s chart and serial negative PCT results, the PharmD questioned the diagnosis of bacterial urosepsis. He noted that the patient list of complaints on admission included pain in her neck and chest area. The PharmD then ordered a troponin and electrocardiogram (EKG) to rule out a myocardial infarction.


Troponin = 1.51 ng/mL, EKG = ST segment elevation

New diagnosis:

ST segment Myocardial Infarction (STEMI)

Treatment, interventions:

Patient received two cardiac stents in the cardiac catheterization lab, antibiotics were discontinued. 

Key takeaways

Lactate is a marker of anaerobic metabolism and has a low sensitivity and specificity for bacterial infection. It should not guide antibiotic therapy. If lactate is elevated with a bacterial infection, the patient is likely septic or in septic shock. Lactate provides information about perfusion not bacterial infection. PCT provides information about bacterial infection, not perfusion.


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