Loss of Smell: Labs, Diagnosis, and Next Steps

Why Smell Loss Deserves a Serious Workup

Losing your sense of smell is not a minor inconvenience. It carries measurable risks: impaired detection of gas leaks and smoke, reduced nutritional intake, higher rates of depression, and an independent association with 5-year mortality. Pinto et al. (2014) followed 3,005 adults aged 57 to 85 and found that those with anosmia had a hazard ratio of 3.37 for death within five years, even after adjusting for age, nutrition, cognitive function, and comorbidities [1].

Despite these stakes, olfactory complaints are frequently dismissed in primary care. A 2016 survey published in Chemical Senses found that fewer than 20% of patients with smell complaints received any objective testing [2]. The gap between symptom prevalence and diagnostic action is wide. An estimated 13.3 million U.S. adults have chronic olfactory dysfunction based on NHANES data, yet most never receive a validated smell test or targeted lab panel [3].

The clinical reality is that smell loss is a symptom, not a diagnosis. Behind every case sits a treatable or at least identifiable cause. The job of the workup is to find it.

Common Causes of Olfactory Dysfunction

Post-viral upper respiratory infections are the single most frequent trigger, responsible for 18% to 45% of cases in large case series [4]. SARS-CoV-2 added an enormous cohort: a 2022 meta-analysis in the BMJ pooled 18 studies and estimated that 5.6% of COVID-19 patients experienced olfactory dysfunction lasting beyond 6 months [5]. Sinonasal disease (chronic rhinosinusitis with or without nasal polyps) accounts for another 15% to 25% of cases [4].

Other causes divide into several categories. Nutritional deficiencies, particularly zinc and vitamin B12, impair olfactory receptor turnover. Head trauma damages olfactory nerve filaments as they pass through the cribriform plate. Medications including ACE inhibitors, methotrexate, and certain antibiotics (macrolides, fluoroquinolones) carry documented olfactory side effects [6]. Neurodegenerative disease is the cause clinicians worry about most: hyposmia precedes motor symptoms of Parkinson disease by 4 to 8 years in up to 90% of patients, per a longitudinal study published in Annals of Neurology [7].

Endocrine disorders round out the differential. Hypothyroidism, Cushing syndrome, and diabetes mellitus all appear in case series of unexplained anosmia [6]. This is why targeted bloodwork matters.

The Validated Smell Test: Where the Workup Begins

Before ordering labs or imaging, clinicians need to quantify the deficit. Subjective self-report correlates poorly with actual olfactory performance. Landis et al. (2003) found that roughly 25% of patients who denied smell problems scored in the hyposmic or anosmic range on formal testing [8].

The University of Pennsylvania Smell Identification Test (UPSIT) is the most widely validated tool. It is a 40-item, forced-choice, scratch-and-sniff test that takes about 15 minutes to administer. Scores are age- and sex-normed, with percentile ranks that classify patients as normosmic, hyposmic (mild, moderate, or severe), or anosmic [9]. The Sniffin' Sticks battery, developed by Hummel et al., is more common in European practice and measures threshold, discrimination, and identification (the "TDI" score) [4].

A brief screening option exists for primary care settings. The Quick Smell Identification Test (Q-SIT) uses just three items and identifies total anosmia with a sensitivity above 95%, though it cannot grade partial loss [9].

Formal testing does two things. It separates true olfactory loss from flavor complaints driven by gustatory dysfunction (patients often say "I can't taste" when the problem is actually smell). It also establishes a baseline against which treatment response can be measured objectively.

Lab Panel: What to Order and Why

No single blood test diagnoses anosmia, but a targeted panel identifies the correctable metabolic and nutritional contributors that clinicians otherwise miss. The following labs are supported by clinical guidelines from the American Academy of Otolaryngology (AAO) and the European Position Paper on Olfactory Dysfunction [4][10].

Zinc (serum or plasma). Zinc is required for carbonic anhydrase VI, an enzyme concentrated in olfactory and gustatory tissue. A 2020 systematic review in Nutrients analyzed 17 studies and concluded that zinc supplementation (30 to 50 mg elemental zinc daily for 3 to 6 months) improved olfactory scores in zinc-deficient patients but offered no benefit to those with normal levels [11].

Vitamin B12 and folate. B12 deficiency causes demyelination of olfactory nerve fibers. Serum B12 below 300 pg/mL with elevated methylmalonic acid warrants supplementation and retesting of smell function after repletion [6].

Thyroid panel (TSH, free T4). Hypothyroidism impairs mucosal turnover in the olfactory epithelium. Correction of the thyroid state has been associated with improved smell scores in small case series [6].

Inflammatory markers (CRP, ESR). Elevated values point toward active inflammatory or autoimmune sinonasal disease (granulomatosis with polyangiitis, sarcoidosis, eosinophilic granulomatosis) [10].

Complete blood count with differential. Eosinophilia suggests allergic or eosinophilic rhinosinusitis. Macrocytic anemia raises suspicion for B12 or folate deficiency.

Fasting glucose or HbA1c. Diabetes-associated microangiopathy can damage the olfactory mucosa. A cross-sectional analysis of NHANES data found that adults with diabetes scored 1.5 points lower on UPSIT than age-matched controls (P <0.001) [3].

In cases where autoimmune etiology is suspected, add ANA, ANCA, and ACE level. For patients with isolated, progressive, or unilateral loss, proceed directly to imaging before waiting for lab results.

Nasal Endoscopy and Imaging: When and What to Order

Nasal endoscopy is the next step after smell testing and basic labs. It visualizes polyps, mucosal edema, masses, septal deviation, and purulent drainage that plain inspection misses. A 2019 AAO clinical practice guideline recommends endoscopy for all patients with anosmia persisting beyond 4 weeks [10].

CT of the paranasal sinuses (non-contrast, coronal cuts) is the imaging standard for sinonasal causes. It quantifies sinus opacification using the Lund-Mackay scoring system, which grades each sinus group from 0 (clear) to 2 (complete opacification). A total score above 12 (out of 24) generally correlates with moderate to severe chronic rhinosinusitis [10].

MRI of the brain and olfactory bulbs is reserved for red-flag scenarios. Order MRI when smell loss is unilateral, when it is progressive over weeks to months without an obvious sinonasal explanation, when it is accompanied by neurological symptoms (tremor, cognitive decline, headaches, visual changes), or when the patient is under 40 with no identifiable cause [4].

MRI can measure olfactory bulb volume. Reduced bulb volume correlates with the severity and duration of olfactory loss. Rombaux et al. (2006) showed that patients with post-viral anosmia had olfactory bulb volumes roughly 50% smaller than healthy controls, and that bulb volume partially recovered in patients who regained smell function after olfactory training [12]. MRI also detects olfactory groove meningiomas, frontal lobe tumors, and early white matter changes associated with neurodegeneration.

Post-Viral Anosmia: The Largest Patient Group

Post-viral olfactory dysfunction (PVOD) accounts for the largest single diagnostic category. The mechanism involves direct viral damage to the olfactory neuroepithelium, which normally regenerates from basal stem cells over 30 to 60 days. When regeneration fails or produces disorganized neural connections, the result is persistent anosmia or parosmia (distorted smell perception) [4].

SARS-CoV-2 differs from other respiratory viruses in its tropism. The virus enters sustentacular (support) cells of the olfactory epithelium via ACE2 receptors rather than directly infecting olfactory neurons. This explains why COVID-related anosmia often resolves faster than post-influenza anosmia. A multicenter European study by Lechien et al. (2021) followed 2,581 COVID-19 patients and found that 85.9% recovered olfactory function within 6 months [13].

For the remaining 14%, the standard recommendation is structured olfactory training. This is not optional or supplementary. It is the primary evidence-based intervention.

Olfactory Training: The Evidence and the Protocol

Olfactory training involves twice-daily, deliberate exposure to four distinct scent categories for a minimum of 12 weeks. The original protocol from Hummel et al. (2009) uses rose (floral), eucalyptus (resinous), lemon (fruity), and cloves (spicy) as the four training odors [14].

The patient sniffs each scent for 10 to 15 seconds, actively concentrating on the memory and identity of the odor. Sessions take under 5 minutes. Compliance matters: patients who trained consistently for 12 or more weeks showed a statistically significant improvement in TDI (threshold, discrimination, identification) scores compared to controls in a randomized trial by Damm et al. (2014, N=144), with a mean improvement of 3.5 TDI points (P=0.002) [15].

A modified protocol rotates to four new odors every 12 weeks. Altundag et al. (2015) compared the classic fixed protocol to a rotating version in 85 patients and found that the rotating group showed greater improvement in odor discrimination scores at 36 weeks (P=0.01) [16].

"We now consider olfactory training a first-line, low-risk intervention for any patient with post-infectious olfactory loss lasting more than two weeks," stated Dr. Thomas Hummel, professor of otorhinolaryngology at Technische Universität Dresden, in the 2017 European Position Paper on Olfactory Dysfunction [4].

Response rates vary by etiology. Post-viral patients respond best (roughly 30% to 60% show clinically meaningful improvement). Patients with congenital anosmia or complete olfactory nerve transection from trauma respond poorly [4].

Medical and Surgical Treatments by Cause

Treatment depends entirely on the underlying diagnosis. No single drug treats "anosmia" as a symptom in isolation.

Chronic rhinosinusitis with nasal polyps. Topical intranasal corticosteroids (fluticasone propionate 200 mcg daily or mometasone 200 mcg daily) are first-line. In a Cochrane review (2011) of 40 trials, topical steroids improved smell scores with a standardized mean difference of 0.45 compared to placebo [17]. For refractory cases with bilateral polyps, dupilumab (300 mg subcutaneously every 2 weeks) reduced nasal polyp score and improved UPSIT by a mean of 10.6 points versus placebo at 24 weeks in the SINUS-24 trial (N=276) [18]. Endoscopic sinus surgery is indicated when medical therapy fails, particularly for patients with Lund-Mackay scores above 12.

Zinc deficiency. Supplementation with 30 to 50 mg elemental zinc daily for 3 to 6 months, then retest [11].

Vitamin B12 deficiency. Intramuscular B12 (1 to 000 mcg weekly for 4 weeks, then monthly) or high-dose oral supplementation (1,000 to 2 to 000 mcg daily). Retesting of smell function at 3 months post-repletion [6].

Hypothyroidism. Levothyroxine titrated to normalize TSH. Smell improvement may lag thyroid normalization by 2 to 3 months [6].

Medication-induced. Drug discontinuation or substitution. ACE inhibitors are the most commonly implicated class in outpatient practice [6].

Head trauma. No pharmacologic intervention has proven effective. Olfactory training is recommended starting 3 months post-injury, once spontaneous recovery plateaus [4].

Neurodegenerative disease. If anosmia is accompanied by REM sleep behavior disorder, constipation, or subtle motor asymmetry, formal neurological evaluation for prodromal Parkinson disease is appropriate. Alpha-synuclein seed amplification assays are now available as a biomarker, though they remain primarily a research tool [7].

When to Worry: Red Flags That Change the Urgency

Most olfactory loss is bilateral, gradual, and tied to a respiratory infection or chronic sinus disease. Certain patterns demand faster action.

Unilateral anosmia. This is a red flag for compressive pathology. Olfactory groove meningiomas classically present with ipsilateral anosmia and contralateral optic changes (Encourage Kennedy syndrome). MRI with gadolinium is the appropriate study [4].

Progressive loss over weeks without URI or sinus disease. This pattern raises concern for neoplasm or inflammatory infiltration (sarcoidosis, granulomatosis with polyangiitis).

Anosmia plus neurological symptoms. Tremor, gait changes, cognitive decline, or personality changes in the context of smell loss should prompt urgent neurological referral. Hyposmia is present in 90% of Parkinson disease patients at the time of motor symptom onset, but it precedes motor findings by years in most cases [7].

Anosmia in a patient under 30 with no obvious trigger. Consider congenital anosmia (Kallmann syndrome if accompanied by hypogonadism, or isolated congenital anosmia). MRI will show absent or hypoplastic olfactory bulbs [4].

Loss of smell after head injury with CSF rhinorrhea. This requires urgent neurosurgical evaluation for cribriform plate fracture.

A Practical Workup Algorithm

The clinical sequence is straightforward. Start with a validated smell test (UPSIT or Sniffin' Sticks) to confirm and grade the deficit. Order the lab panel (zinc, B12, folate, TSH, free T4, CRP, ESR, CBC, HbA1c). Perform nasal endoscopy. If endoscopy is unremarkable and labs are normal, initiate olfactory training for 12 weeks while monitoring for red-flag features.

"In the absence of red flags, a 12-week trial of olfactory training before advanced imaging is a reasonable and cost-effective strategy," per the 2019 AAO clinical practice guideline on adult sinusitis [10].

If red flags are present (unilateral loss, progressive course, neurological signs), skip to MRI. If endoscopy reveals polyps or mucosal disease, obtain CT sinuses and begin medical therapy. Retest with the same validated smell test at 12 weeks to measure response. Persistent anosmia after 6 months of appropriate treatment warrants referral to an otolaryngologist or a dedicated smell and taste clinic, which are available at roughly 40 academic medical centers in the United States.