Q: My 2-year-old had a 4-day fever, low platelets, and very high Vitamin B12 on a blood test. Is this normal after a virus, or should we check for Kawasaki or leukemia?

Medically reviewed by Sang Hyun Ahn, MD

Content edited by Myeongchan Kim, MD

This post is a dramatization of a question and answer session experienced by our actual medical reviewers. Please note that this content is for informational purposes only and does not constitute medical advice. Always consult with a qualified healthcare professional for any health concerns or before making any decisions related to your health or treatment.

Question:

My 2-year-old child recently had a high fever for four days. We visited the clinic daily for IV fluids.

• Day 1: 39.4°C, prescribed medication.

• Day 2: 39°C range, dropped to high 38°C with alternating antipyretics. Had blood tests and IV fluids.

• Day 3: Peak 39°C, dropped to low 38°C with IV fluids.

• Day 4: Peak 38.5°C, temperature normalized after IV fluids.

• Day 5: Fever broke, morning rash appeared.

The Day 2 blood test showed WBC 8.4, Platelets (PLT) 103, and CRP 3.49. Tests for UTI, stool, flu, and COVID-19 were all negative. The doctor noted the inflammation marker (CRP) was surprisingly high since there was only mild throat swelling and no other cold symptoms. They considered Kawasaki disease and meningitis but couldn't diagnose Kawasaki since the fever didn't last full 5 days.

I'm very worried because the platelets were low (103), and I remember the IV site took a long time to stop bleeding. Two days after the fever broke, another blood test showed that while other numbers were fine, Vitamin B12 was extremely high (in the thousands). My child doesn't take B-vitamin supplements.

When I searched for causes of high Vitamin B12, I found frightening possibilities like liver/kidney issues or leukemia. Why would Vitamin B12 be so high? Should we repeat the blood test or visit a larger hospital? Also, should we get an echocardiogram just in case it was atypical Kawasaki disease?

Answer:

It is completely understandable that you are worried after your child experienced a four-day high fever followed by a rash and abnormal blood test results.

Current Situation Assessment:

• Roseola Infantum: The pattern of a high fever breaking followed by a sudden rash is a classic presentation of Roseola (Exanthema Subitum), a very common viral infection in toddlers.

• CRP 3.49: This level of C-reactive protein (an inflammation marker) is frequently seen during acute viral infections.

• Platelets (PLT) 103: While this is below the normal lower limit (150), it is very common for platelet counts to temporarily drop during a viral infection. They typically rise back to normal levels naturally during the recovery phase.

• High Vitamin B12: During acute infections and systemic inflammation, Vitamin B12 can temporarily spike due to its rapid release from the liver or minor cellular destruction. This is often a transient response to the virus rather than a sign of a chronic underlying disease.

What You Can Do at Home:

• Take clear pictures of the rash daily to track its progression.

• Ensure your child stays well-hydrated and gets plenty of rest.

• Monitor for any new symptoms, such as the return of a fever, unusual bleeding or bruising, swollen lymph nodes, or a swollen abdomen.

When to Seek Further Evaluation:

In most cases, these blood abnormalities are a temporary reaction to the viral infection. However, you should discuss a follow-up plan with your pediatrician. Consider asking them about scheduling a repeat blood test in about two weeks to confirm that the platelets, CRP, and Vitamin B12 have returned to normal.

Regarding Kawasaki disease, if your child did not exhibit other classic signs (such as severe red "strawberry" tongue, cracked/red lips, swollen hands/feet, or significantly enlarged lymph nodes), an echocardiogram may not be strictly necessary. However, your pediatrician is the best person to make that clinical judgment based on an in-person physical exam. A referral to a larger hospital is generally only considered if abnormal blood counts persist after two weeks, or if new warning signs appear.

Warning Signs to Watch For:

Seek immediate in-person medical evaluation if you notice any of the following:

• The fever returns along with significant lethargy.

• Worsening bruising or bleeding tendencies (like frequent nosebleeds or gum bleeding).

• Abdominal pain or swelling.

• Lymph nodes that continue to enlarge.

Summary:

The most likely cause of your child's symptoms is Roseola, and the fluctuations in platelets and Vitamin B12 are frequently temporary post-viral changes. Please consult your pediatrician to schedule a follow-up blood test in about two weeks to ensure all values have normalized and to gain peace of mind.

For extra peace of mind and clear guidance when tracking your child's symptoms, the FeverCoach app is always there to help.

fevercoach.us

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