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Viral loads and travel-related viruses

15 min

  • 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

Read the full reference

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?

The next section explains it.

Right. The next section explains why.

The next section explains it.

The next section explains it.

Get the idea

A viral load changes on a log scale

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 counts 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.

Only comparable results share a trend. 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 decides what they mean for the patient.1

Detected below quantification is a result

An assay can detect a target at a level too low to measure. That result says the virus is present. It is reported as detected below the lower limit of quantification (LLOQ), with the limit stated. Zero and the limit value itself are numbers the assay never measured.2 A laboratory information system field that accepts only numbers needs a supported text result for it.3

A traveler's test depends on the day of illness

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

Illness dayTest on serumWhat a result means
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 sorting out.4 A febrile traveler from a malaria-risk area also needs urgent malaria testing.5

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

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.

    Only results from one specimen type, assay and laboratory can share a trend.

  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. Pick 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 counts only when it passes 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.

Your turn

Problem 1 of 3

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

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 share 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. Ask whether any equivalence between the two methods has been shown.

Review CMV results after a change of laboratory

Problem 3 of 3

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 says no virus was found. The assay found 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 loses 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 keeps the qualitative finding and the reporting limit. The report needs a field that can carry text for it.

Review Detection below quantification

Use it

  • Priya Raman, 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

What dengue testing fits her serum today?

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

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

Days 0 to 7 pair a direct test with IgM. IgM can start to appear as RNA and NS1 fall, 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 that.

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 without proving 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 pinned down. What do you suggest?

A second IgM carries 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 name yellow fever.

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

Review Arboviral and travel-related testing

The clue that settled this case is the illness day, together with her flavivirus history. Day 4 called for a direct test paired with IgM. Her yellow fever vaccination kept one positive IgM presumptive.

Keep

Sources checked