Aplisol and Tubersol are the two tuberculin skin test products available in the United States, and while both contain purified protein derivative (PPD) from the same bacterium, they are not perfectly interchangeable. In studies of people recently diagnosed with tuberculosis, the two products produced median reactions within about a millimeter of each other and agreed on positive-or-negative status in virtually every case when the same cutoff was applied. Yet switching from one product to the other in a workplace screening program has repeatedly caused a spike in apparent new positives that turn out to be false alarms. That gap between controlled comparisons and real-world screening behavior is the core of the Aplisol-vs.-Tubersol question, and it matters more for institutional policy than most clinicians realize.
What These Two Products Actually Are
Both Aplisol (made by Par Pharmaceutical, now Endo) and Tubersol (made by Sanofi Pasteur) deliver tuberculin PPD into the skin of the forearm using the Mantoux technique. Each is standardized against a reference preparation called PPD-S, which the U.S. FDA uses as the benchmark for commercial tuberculin. A standard dose of 5 tuberculin units is drawn into a syringe, injected just under the surface of the skin, and read 48 to 72 hours later by measuring the diameter of any hard, raised bump (induration) that forms. The test works because memory T cells in someone previously exposed to tuberculosis recognize the injected protein fragments, triggering a localized immune reaction that produces the measurable swelling.
Despite starting from the same bacterial source and targeting the same immune response, the two products differ in their manufacturing processes, stabilizers, and the exact protein fractions that end up in the final vial. Those differences are subtle enough that head-to-head studies in confirmed TB patients show near-identical sensitivity. In one direct comparison, Aplisol reactions had a median size of 16 mm and Tubersol reactions a median of 15 mm, with the average paired difference coming in well under a millimeter and no statistical difference between them.1PubMed. Comparison of sensitivities to two commercially available tuberculin skin test reagents in persons with recent tuberculosis At a 10 mm cutoff, both products classified 96% of confirmed TB patients as positive, and at a 5 mm cutoff, sensitivity was 100% for both.
Why Switching Products Causes Problems
If the two products perform so similarly in confirmed cases, you might expect switching between them to be seamless. It is not. The clearest evidence comes from healthcare worker screening programs, where large groups of employees get tested annually with the same product, then the institution changes suppliers. When one hospital system switched from Tubersol to Aplisol in 2004 after years of using Tubersol, the annual conversion rate jumped from a range of 0.3–0.9% to 2%, a statistically significant spike. Of the 24 workers flagged as new converters under Aplisol, only about a quarter tested positive when retested with Tubersol, and none of them had chest X-ray evidence of actual TB infection.2PubMed. Increase in tuberculin skin test converters among health care workers after a change from Tubersol to Aplisol When the results were reclassified based on Tubersol retesting, conversion rates fell back to their historical range.
A similar pattern showed up at a Veterans Administration facility. Among employees who were negative on a blood test for TB, nearly all reacted to Aplisol on skin testing while none reacted to Tubersol.3PubMed Central. Differences in tuberculin reactivity as determined in a veterans administration employee health screening program The implication was clear: Aplisol was triggering reactions in people who almost certainly did not have TB. The researchers concluded that switching between tuberculin preparations could falsely inflate the number of positive results, echoing CDC guidance that institutions should stick with one product rather than bouncing between them.
The practical fallout of these false positives is considerable. An employee flagged as a new converter typically needs a chest X-ray, a medical evaluation, and potentially months of preventive antibiotic treatment. Multiply that across dozens of false converters in a hospital system and you get unnecessary radiation exposure, wasted clinic time, employee anxiety, and drug side effects that nobody needed. The cost is not just financial; it erodes trust in the screening program itself.
Why Aplisol Tends to Read Hotter
The exact reason Aplisol produces slightly larger reactions in some populations is not fully settled, but the leading explanation involves differences in the non-tuberculin protein fractions that survive each manufacturer’s purification process. Tuberculin PPD is not a single molecule. It is a complex mixture of proteins extracted from heat-killed Mycobacterium tuberculosis cultures. Aplisol and Tubersol use different filtration and concentration steps, and the proportions of various protein fragments in the final product are not identical. Some of those fragments can cross-react with immune memory from environmental mycobacteria or prior BCG vaccination, which means a product with a broader protein profile can trigger bigger bumps in people who have never actually had TB.
One comparison that sheds light on this involved testing Tubersol and Aplisol alongside the original reference standard, PPD-S1, in a low-risk population with no TB history, no known exposures, and no BCG vaccination. Tubersol produced slightly smaller reactions than Aplisol or the reference standard, but the specificities of both commercial products were comparably high, around 98%.4Journal of Clinical Tuberculosis and Other Mycobacterial Diseases. Tuberculin test using Indian indigenous purified-protein derivative (PPD) shows only moderate agreement with international standard PPD In other words, when the population had minimal cross-reacting exposures, the two products behaved nearly the same. The differences emerge most sharply in populations with more immunologic “noise” from other mycobacterial encounters.
How the Skin Test Reaction Works
Understanding why two slightly different protein mixtures can give different answers requires knowing what happens under the skin after injection. The tuberculin test is a delayed-type hypersensitivity response. When PPD is injected, antigen-presenting cells in the skin pick up the protein fragments and display them. If the person has been previously exposed to TB, memory T cells recognize those fragments, move to the injection site, and begin multiplying and releasing signaling molecules that attract more immune cells, primarily macrophages.5PubMed. Immunopathogenesis of delayed-type hypersensitivity The swelling and hardness you feel at the test site are the physical result of all those cells accumulating and the local tissue inflammation they produce.6PubMed. Mantoux Test as a model for a secondary immune response in humans
The reaction peaks at 48 to 72 hours, which is why readings are scheduled in that window. Memory T cells are the immunologically active players. Macrophages serve as both the initial alarm system and the main effector cells once signaled by T cell products.7PubMed. Mechanism of a reaction in vitro associated with delayed-type hypersensitivity The size of the induration reflects how many memory T cells recognized the injected proteins, and a product containing a broader assortment of mycobacterial proteins can recruit a wider pool of cross-reactive memory cells, making the bump bigger even when TB was never involved.
BCG Vaccination and Why It Muddies the Picture
The Bacille Calmette-Guérin (BCG) vaccine, given at birth or in early childhood in much of the world, trains the immune system using a live but weakened mycobacterium closely related to the TB bacterium. That training means the immune system of a BCG-vaccinated person may react to tuberculin PPD even if they have never encountered actual TB. In children vaccinated in the first two months of life, average induration sizes remain significantly higher than in unvaccinated children, and this effect persists at least through age six.8PubMed. Effect of BCG vaccine on tuberculin skin tests in 1-6-year-old children
A longer-term follow-up found that BCG vaccination given after infancy more than doubled the risk of a positive skin test in the first 15 years, and even 16 to 55 years later, a modest effect remained.9PubMed. The Long-term Effect of Bacille Calmette-Guérin Vaccination on Tuberculin Skin Testing: A 55-Year Follow-Up Study This is relevant to the Aplisol-vs.-Tubersol question because BCG-vaccinated people already have a higher baseline of cross-reactive T cells. A product that elicits slightly larger reactions can push those individuals across the positive threshold more readily, amplifying the discordance between the two products in diverse populations where BCG is common.
Environmental Mycobacteria Add More Noise
BCG is not the only source of cross-reactivity. Non-tuberculous mycobacteria (NTM) are widespread in soil and water worldwide, and casual exposure can sensitize a person’s immune system enough to produce a small tuberculin reaction. Across 18 studies covering over a million people, the rate of false-positive skin tests attributable to NTM cross-reactivity ranged from less than 1% to about 2%, depending on the region.10PubMed. False-positive tuberculin skin tests: what is the absolute effect of BCG and non-tuberculous mycobacteria? In children, NTM sensitization has been shown to cause false positives on the skin test that can lead to unnecessary preventive treatment, and blood-based interferon-gamma tests can help sort out who truly needs treatment.11European Respiratory Journal. Evaluating the non-tuberculous mycobacteria effect in the tuberculosis infection diagnosis
NTM exposure varies dramatically by geography and climate. Someone living in the southeastern United States or in tropical regions encounters more environmental mycobacteria than someone in a dry, cold climate. That means the background false-positive rate from NTM is not uniform, and any product-level difference in how broadly the tuberculin proteins cross-react with NTM-driven memory cells will show up more in high-NTM regions.
Blood Tests as an Alternative
Given all the noise from BCG, NTM, and product-level variation, many institutions have moved toward interferon-gamma release assays (IGRAs) like QuantiFERON-TB Gold and T-SPOT.TB. These blood tests measure the immune response to TB-specific proteins that are absent from BCG and most environmental mycobacteria, which sharply reduces cross-reactivity. In a large comparison, both IGRA platforms predicted progression to active TB disease significantly better than the skin test did.12JAMA Network Open. Comparison of Tuberculin Skin Testing and Interferon-γ Release Assays in Predicting Tuberculosis Disease
Agreement between the skin test and IGRAs is moderate at best, particularly in children. One pediatric study found that among children classified as having latent TB by the skin test, only about half also tested positive on an IGRA.13PLoS ONE. A Three-Way Comparison of Tuberculin Skin Testing, QuantiFERON-TB Gold In Tube and T-SPOT.TB in Children Another found overall sensitivity and specificity of roughly 66% and 82%, respectively, when comparing the two approaches in children.14PubMed Central. Comparison of quantiferon test with tuberculin skin test for the detection of tuberculosis infection in children The discordance is highest in BCG-vaccinated populations, exactly where the Aplisol-vs.-Tubersol question matters most. For institutions that find themselves needing to switch tuberculin suppliers, moving to an IGRA instead sidesteps the product-switching problem entirely.
Supply Shortages and Forced Switches
The reason institutions switch products in the first place is usually not by choice. Tuberculin shortages have hit the U.S. market repeatedly. When Sanofi Pasteur experienced production problems with Tubersol, healthcare providers scrambled to buy Aplisol, creating secondary shortages of that product as well.15PubMed Central. National shortage of purified-protein derivative tuberculin products By mid-2013, public health officials in more than half of U.S. jurisdictions reported shortages of at least one tuberculin product severe enough to interrupt their screening activities.16PubMed Central. Extent and effects of recurrent shortages of purified-protein derivative tuberculin skin test antigen solutions – United States, 2013
These shortages put infection-control teams in a bind. CDC guidance says to stick with one product, but when that product is unavailable for months, the only alternative is to switch or to suspend screening entirely. Some programs used the disruption as an opportunity to migrate permanently to IGRAs. Others switched tuberculin brands and then dealt with the expected bump in apparent conversions, sometimes retesting flagged workers with the original product once supply returned. Neither option is seamless, and the recurring nature of these shortages means that most large healthcare systems in the U.S. have confronted this dilemma at some point in the past two decades.
Measurement Variability Between Readers
Even when the same product is used consistently, the skin test carries built-in imprecision from the human element of reading the result. Two nurses measuring the same induration can disagree by several millimeters. One study found that a second measurement by a different observer could differ by as much as 3 to 4 mm in either direction, enough to reclassify a borderline positive as negative or vice versa.17PubMed. Reliability of tuberculin skin test measurement In children, the disagreement was most pronounced when the actual induration fell close to the clinically significant cutoff, with a tendency toward under-reading.18PubMed. Mantoux test revisited: Variability in reading tuberculin test in pediatric population
This variability is worth keeping in mind when evaluating Aplisol-vs.-Tubersol differences. If the average gap between the two products is around 1 mm but reader disagreement can span 3 to 4 mm, the product choice matters less for any individual test than the skill and consistency of the person measuring the bump. For serial screening programs tracking year-over-year changes, though, both sources of variation compound each other, which is why product switching and reader inconsistency together can produce misleading trends.
When the Immune System Cannot Respond
All tuberculin skin tests, regardless of brand, share a fundamental limitation: they require a functioning immune system to produce a readable reaction. In people with advanced HIV infection, the T cells that drive the delayed-type hypersensitivity response are depleted. One study among intravenous drug users found that HIV-positive individuals were about half as likely as HIV-negative individuals to test positive on a tuberculin test, even using a lowered cutoff designed to improve sensitivity. The deficit worsened as CD4 cell counts dropped, and skin test anergy to unrelated antigens confirmed that the immune system simply was not responding.19JAMA. Prevalence of Tuberculin Positivity and Skin Test Anergy in HIV-1—Seropositive and —Seronegative Intravenous Drug Users
In this population, the Aplisol-vs.-Tubersol question is largely irrelevant because neither product can reliably trigger a reaction. Blood-based IGRAs face their own performance challenges in immunosuppressed patients, but they at least do not rely on an intact skin-level immune response. For anyone with significant immune suppression from HIV, organ transplant medications, or chemotherapy, the choice of tuberculin brand is the wrong question. The right question is whether a skin test is the appropriate tool at all.
Storage and Handling Can Override Product Choice
A perfectly manufactured vial of either product can lose potency if mishandled. Tuberculin PPD should be stored refrigerated and protected from light. Quality-control studies of PPD products from Latin American laboratories found that potency held for six months after dilution when stored at refrigerator temperatures, but vials that were repeatedly opened and stored with less than half their original volume should be discarded.20Journal of Biological Standardization. The quality control of tuberculin PPD products from seven Latin American laboratories Under more extreme conditions, though, tuberculin PPD proved surprisingly tough. Prolonged testing found that only very long exposure to extreme heat compromised the biological activity of the preparations.21Journal of Infection in Developing Countries. The stability of human, bovine and avian tuberculin purified protein derivative (PPD)
In field settings, the practical concern is not so much the product sitting on a truck in summer for a day, but rather repeated drawing from the same vial over weeks without refrigeration. For clinic managers comparing Aplisol and Tubersol, ensuring proper cold-chain handling of whichever product they use matters more for result accuracy than the brand name on the label.
The Same Problem in Cattle
Product-level variability in tuberculin testing is not unique to humans. In bovine tuberculosis control, the brand of PPD used for skin testing in cattle significantly influences results. A study in Ecuador found that different commercial PPD brands produced varying responses in tuberculous cattle, raising the possibility of missed diagnoses depending on which product was used.22Scientific Reports. Assessing the impact of various tuberculin PPD brands on bovine tuberculosis diagnosis And in naturally infected cattle subjected to repeated skin testing at short intervals, the skin test response progressively weakened over successive rounds, suggesting that repeat testing itself can desensitize the immune reaction and lead to false negatives.23PubMed Central. Repeat tuberculin skin testing leads to desensitisation in naturally infected tuberculous cattle which is associated with elevated interleukin-10 and decreased interleukin-1 beta responses
The veterinary parallel underscores a broader point: tuberculin PPD is an inherently imperfect diagnostic tool, whether used in humans or animals. The protein mixture approach was a breakthrough when it was developed over a century ago, and it remains useful because it is cheap, requires no laboratory, and works in remote settings. But its limitations, from product-level variability to reader subjectivity to immune interference, are baked into the biology of what it measures. The Aplisol-vs.-Tubersol distinction sits inside this larger story as one more source of variation in a test that was never as precise as a binary positive-or-negative result implies.
International PPD Standards and Cross-Border Confusion
Outside the United States, neither Aplisol nor Tubersol is the default product. Most of the world uses RT 23, a tuberculin preparation produced by the Statens Serum Institut in Denmark and distributed by the WHO. An early head-to-head comparison found that 2.5 tuberculin units of RT 23 (in a Tween 80 diluent) was roughly equivalent to 5 tuberculin units of PPD-S in people with true TB sensitivity. Among people sensitized by environmental mycobacteria, however, RT 23 produced considerably more reactions than PPD-S.24PubMed Central. A COMPARISON IN THE UNITED STATES OF AMERICA OF TWO TUBERCULINS, PPD-S AND RT 23 This means that someone screened overseas with RT 23 and retested in the U.S. with Tubersol or Aplisol could get a different result even if nothing about their TB status has changed, purely because the protein mixtures differ.
Travelers, immigrants, and international healthcare workers frequently encounter this mismatch. A positive tuberculin result obtained abroad with RT 23 is not directly comparable to a result obtained in the U.S. with Aplisol or Tubersol, especially when the person has a BCG vaccination history. Clinicians who interpret these results need to know which product was used, and in many cases the information simply is not documented. For this population, an IGRA blood test, which does not vary by brand or by BCG status, often provides a cleaner answer.