Hospitalists ordering diagnostic tests face a persistent challenge: interpreting results that don’t always tell a clear story, especially in older or complex patients. A series of case-based articles from The Hospitalist tackles this problem head‑on, walking through common scenarios where lab values can mislead or require deeper context. The collection covers tests from cystatin C and pyuria to serum ammonia and D-dimer, each grounded in real patient cases.
When Kidney Function Tests Miss the Mark
One entry, published in October 2025, focuses on cystatin C for assessing kidney function in hospitalized patients. The case involves a 68-year-old man on chronic steroids for amiodarone‑induced thyroiditis who arrived with pneumonia, hypotension, and encephalopathy. His pneumonia resolved, but his confusion worsened.
The piece walks through how standard creatinine‑based estimates of kidney function can be unreliable in patients with low muscle mass or certain medications. Cystatin C, produced at a constant rate by all nucleated cells, isn’t affected by muscle mass or diet the same way creatinine is. In this scenario, cystatin C levels gave clinicians a more accurate picture of the patient’s true kidney function, which helped guide medication dosing and fluid management.
The implication for hospitalists is practical.
A patient who appears stable on creatinine might actually have significant impairment that only cystatin C reveals. That matters when dosing antibiotics or contrast for imaging.
Pyuria in the Elderly: Not Always an Infection
A June 2025 case examines pyuria — white blood cells in the urine — in geriatric patients. A 75-year-old woman from a nursing home with mild cognitive impairment was admitted for two days of confusion, agitation, and weakness. She denied any urinary symptoms.
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Urinalysis showed pyuria. The authors make a careful point: in older adults, pyuria can be present for reasons other than a urinary tract infection. Chronic inflammation, indwelling catheters, vaginal contamination, or recent antibiotic use can produce white cells in the urine. Treating every positive urinalysis with antibiotics in this population leads to overuse, resistance, and side effects.
Context matters more than the lab value alone.
Without symptoms like dysuria or frequency, pyuria in a confused elderly patient shouldn’t automatically trigger treatment. The real question is whether the urinary tract is the source of the confusion — and often, it isn’t.
Ammonia and Confusion: A Common Misstep
A December 2024 case features a 77-year-old woman with hepatitis C cirrhosis who arrived with progressive confusion over 24 hours. Serum ammonia was ordered.
They note that while high ammonia supports a diagnosis of hepatic encephalopathy, the test has significant limitations. Ammonia levels can be normal in some patients with encephalopathy, and high levels don’t always correlate with symptom severity. More importantly, mishandling the blood sample — leaving it at room temperature too long — can push the number higher than the patient’s true level.
For hospitalists, the takeaway is straightforward: ammonia is a supportive test, not a definitive one.
Clinical judgment, combined with history and physical exam findings, remains the foundation for diagnosing hepatic encephalopathy.
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D-dimer and Pulmonary Embolism: Know When to Use It
A February 2024 case describes a 58-year-old previously healthy woman with pleuritic chest pain after moving heavy boxes. She had no dyspnea, leg swelling, recent surgery, or immobility. The question was whether a D-dimer should be ordered.
The entry explains that D-dimer is most useful in patients with low or moderate pretest probability for pulmonary embolism. In this situation, the patient’s Wells score was low, making a negative D-dimer reliable enough to rule out PE without imaging. But they caution that D-dimer specificity drops with age, pregnancy, and hospitalization itself. A positive result in an older or hospitalized patient often leads to unnecessary CT scans and contrast exposure.
The case reinforces a basic but easily forgotten rule: test selection matters as much as test interpretation. Ordering a D-dimer in a patient with high pretest probability doesn’t help — it just creates delay and radiation.
A 2023 case on MRSA nasal PCR looks at a 67-year-old man with COPD admitted from a nursing home for pneumonia. His MRSA nasal swab came back negative. The writers argue that a negative MRSA nasal PCR has a high negative predictive value for MRSA pneumonia, meaning empiric vancomycin can often be stopped safely. That matters for reducing nephrotoxicity and antibiotic pressure.
The collection also covers venous blood gas as an alternative to arterial blood gas, interferon‑gamma release assays for tuberculosis, beta‑D‑glucan for fungal infections, and urine drug screen interpretation. Each case follows the same format: a real patient scenario followed by practical guidance on test limitations.
Diagnostic tests are tools, not answers. A lab value pulled from the chart without understanding the patient’s physiology, age, medications, or sample handling is just a number. Hospitalists who read these cases might find themselves ordering fewer tests — and trusting the ones they do order a little more.
