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Viral loads and travel-related viruses

15 min

Skills you'll build3 skills
  • Calculate a serial viral-load change in log₁₀ and compare it with assay variability
  • Distinguish a result detected below quantification from a negative viral result
  • Choose NAAT, NS1 antigen, or serology for a returning traveler by day of illness

Try first

Try first

A transplant recipient's plasma cytomegalovirus (CMV) DNA is 2,000 IU/mL, and a week later 5,000 IU/mL on the same assay. How large is the change, and is it significant?

Keep going to see the reasoning.

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

Comparing serial viral loads

Quantitative CMV results are compared as a ratio on a log₁₀ scale:

log₁₀ change = log₁₀(later result ÷ earlier result)

The 2025 international consensus on CMV in solid organ transplantation treats a change as significant only when it exceeds 0.5 log₁₀. When the earlier value is below 1,000 IU/mL, the change must exceed 0.7 log₁₀, because assays vary more near the low end.1 A 0.5 log₁₀ change is about a threefold change, and 0.7 log₁₀ is about fivefold.

A trend can be read only from comparable results. Plasma and whole blood give different values, and two assays can differ even when both report IU/mL. Serial results come from one specimen type, one assay and one laboratory whenever possible.1 The laboratory reports the values and the measured change, and the transplant service interprets them for the patient.1

Results below the limit of quantification

An assay can detect a target at a level too low to measure. Such a result shows the virus is present. It is reported as detected below the lower limit of quantification (LLOQ), with the limit stated. Neither zero nor the limit value was measured.2 A laboratory information system field that accepts only numbers needs a supported text result.3

Dengue testing by day of illness

Early in dengue the virus is detectable in blood, and antibody appears later.4

Illness dayTest on serumInterpretation
0 to 7NAAT or NS1 antigen, each with IgMPositive NAAT or NS1 confirms infection. A negative direct test leaves dengue possible
After 7IgMOne positive IgM is presumptive

Flavivirus antibodies cross-react after other flavivirus infections and after vaccination against yellow fever or Japanese encephalitis. The public health laboratory can run neutralization testing when an IgM result needs resolution.4 A febrile traveler from a malaria-risk area also needs urgent malaria testing.5

Follow the bench demonstration

Sources for this step
References
  1. Kotton CN, Kumar D, Manuel O, et al. The Fourth International Consensus Guidelines on the Management of Cytomegalovirus in Solid Organ Transplantation. Transplantation. 2025;109(7):1066-1110. doi:10.1097/TP.0000000000005374
  2. Quest Diagnostics. Hepatitis C virus antibody and RNA testing. FAQ22v1. Accessed September 27, 2026. https://www.questdiagnostics.com/healthcare-professionals/clinical-education-center/faq/faq22v1
  3. Standard: test report. 42 CFR §493.1291. Accessed September 27, 2026. https://www.law.cornell.edu/cfr/text/42/493.1291
  4. Centers for Disease Control and Prevention. Clinical testing guidance for dengue. Accessed September 27, 2026. https://www.cdc.gov/dengue/hcp/diagnosis-testing/
  5. Centers for Disease Control and Prevention. Evaluation and diagnosis of malaria. Accessed September 27, 2026. https://www.cdc.gov/malaria/hcp/clinical-guidance/evaluation-diagnosis.html

Watch one

Read each decision and its explanation.

A kidney transplant recipient is monitored with plasma CMV DNA on the center's assay. Last week's result was 600 IU/mL. Today's is 3,300 IU/mL. Both are within the assay's quantitative range. Is the rise significant?

  1. Check comparability: both are plasma, on the same assay, in the same laboratory, within the quantitative range.

    A trend can be read only from one specimen type, assay and laboratory.

  2. Take the ratio: 3,300 IU/mL ÷ 600 IU/mL = 5.5.

    A log change starts from the ratio of the two results.

  3. Convert: log₁₀ 5.5 = 0.74 log₁₀.

    The consensus limits are written in log₁₀ units.

  4. Select the limit: the earlier value, 600 IU/mL, is below 1,000 IU/mL, so the change must exceed 0.7 log₁₀.

    Assays vary more near the low end, so a low starting value needs a larger change.

  5. Compare: 0.74 log₁₀ exceeds 0.7 log₁₀.

    A change is significant only when it exceeds the limit.

The rise is 0.74 log₁₀, which exceeds the 0.7 log₁₀ limit for a starting value below 1,000 IU/mL. The laboratory reports both values and the measured rise, and the transplant service interprets them.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

Two comparable plasma CMV results from the same assay are 400 IU/mL and, a week later, 1,600 IU/mL. How is the change interpreted under the 2025 solid-organ-transplant consensus?

Correct. log10(1,600 ÷ 400) = log10 4 = 0.60. A significant change must exceed 0.5 log10, or 0.7 log10 when the initial value is below 1,000 IU/mL, and 400 IU/mL is below 1,000. The laboratory reports the measured values and trend, and the transplant service interprets them.

Incorrect. The 0.5-log10 limit applies when the initial value is 1,000 IU/mL or more. Below 1,000 IU/mL the change must exceed 0.7 log10, because assay variability is greater near the lower end of measurement.

Incorrect. A log change compares the two concentrations as a ratio, log10(later ÷ earlier). The log of their difference only restates the 1,200 IU/mL difference as a concentration.

Hint
  1. Divide the later result by the earlier one, then take log₁₀.
  2. Check whether the first value is above or below 1,000 IU/mL before choosing the limit.

Review Interpreting a serial CMV change

Problem 2 of 3 · Hints available

A transplant center's plasma CMV assay reports 2,500 IU/mL. An outside laboratory's whole-blood assay reports 25,000 IU/mL a week later. What does the scientist conclude?

Incorrect. Dividing 25,000 by 2,500 gives 10. Whole blood includes cell-associated DNA, and the two assays are not interchangeable, so that ratio mixes a matrix and method difference with any biological change.

Correct. Whole blood includes cell-associated DNA, and assays differ even when both report IU/mL, so without documented equivalence the values cannot be plotted on one trend. The outside result is retained with its method and specimen identified, and follow-up uses the center's plasma method.

Incorrect. No conversion factor between these matrices and assays is validated, and the result stays identifiable as a whole-blood measurement.

Hint
  1. Compare the specimen types and the laboratories behind the two numbers.
  2. Consider whether the two methods have been shown to be equivalent.

Review CMV results after a change of laboratory

Problem 3 of 3 · On your own

A hepatitis B virus DNA assay with an LLOQ of 10 IU/mL flags a patient's plasma as "Detected, below 10 IU/mL." The result field in the laboratory information system accepts only numbers. What goes on the report?

Zero would mean no virus was detected. The assay detected it below the level it can measure.

Replaced a detected result below quantification with zero

A result detected below the lower limit of quantification shows the target is present at a level the assay cannot measure. Reporting it as zero, or as the limit value, turns a qualitative finding into a number the assay never produced.

The assay detected the target. Reporting it as not detected omits a positive finding.

10 IU/mL is the assay's limit. The patient's level was below it and was never measured.

Replaced a detected result below quantification with zero

A result detected below the lower limit of quantification shows the target is present at a level the assay cannot measure. Reporting it as zero, or as the limit value, turns a qualitative finding into a number the assay never produced.

This gives the qualitative finding and the reporting limit. The report needs a field that accepts text.

Review Detection below quantification

Use it

Read the case evidence, then make each decision. The step is complete without it.

  • Priya Pelletier, 29, a marine biology graduate student, returns from 3 weeks of reef fieldwork in Puerto Rico during a dengue outbreak.
  • She has fever, headache and a rash. Her illness began 4 days ago. She is not pregnant.
  • She grew up in Brazil and received yellow fever vaccine as a child.
Decision 1 of 3

Which dengue testing is appropriate for her serum today?

On illness day 4, IgM can still be absent. A direct test detects the virus this early.

A single IgG result cannot show a current infection, and her vaccine and childhood exposures can raise flavivirus IgG.

On days 0 to 7 a direct test is paired with IgM. IgM begins to appear as RNA and NS1 decline, so the pair covers both.

Review Arboviral and travel-related testing

Decision 2 of 3

Dengue NAAT is not detected, and dengue IgM is positive. How is this reported?

One positive IgM is presumptive. Flavivirus antibodies cross-react, and her yellow fever vaccination adds to the cross-reactivity.

Reported one flavivirus IgM as a confirmed infection

A single positive dengue IgM is presumptive, because flavivirus antibodies cross-react after other infections or vaccination. Reporting it as confirmed can name the wrong virus. NAAT or NS1 antigen testing early in illness, and neutralization testing where it can resolve cross-reactions, identify the virus more reliably.

A positive IgM with a negative direct test supports recent infection and does not identify which flavivirus caused it.

A negative direct test leaves dengue possible, and her IgM is positive.

Review Arboviral and travel-related testing

Decision 3 of 3

The clinician asks how the virus could be identified. What do you suggest?

A second IgM has the same cross-reactivity as the first.

Her vaccination was years ago, and a current illness after travel needs its own explanation. The IgM result does not identify yellow fever.

Neutralization testing can resolve some cross-reacting IgM results. After earlier flavivirus exposure it may still be inconclusive.

Review Arboviral and travel-related testing

The testing depends on the day of illness and her flavivirus history. Day 4 calls for a direct test paired with IgM. Her yellow fever vaccination means one positive IgM stays presumptive.

For reference

Sources checked