This is a real case, with patient information altered to maintain anonymity.
I recently saw an 10-year-old boy in clinic for evaluation of hypertension. He has type 1 diabetes that was diagnosed at age 2, which has been poorly controlled and requiring multiple admissions for hyperglycemia, a couple times associated with DKA. His BPs at prior outpatient clinic visits had been normal, but during his most recent admission he had automated systolic BPs up to the 140s-150s mmHg, but with manual auscultation they were in the 130s mmHg range. He had no hypertensive symptoms. He is tall for age (>97th percentile) but also obese (BMI 98th percentile). He had normal renal function (serum creatinine 0.5 mg/dL). A renal ultrasound was obtained while he was inpatient, which was normal.
When I saw the patient in clinic, he had a normal manual blood pressure of 109/66 and normal four extremity BPs. Due to his obesity he required an adult-sized BP cuff. Based on the new 2017 AAP Pediatric Hypertension guidelines with updated BP tables, his 90th percentile was 113/75, 95th percentile was 118/78, and 95th + 12 mmHg was 130/90.
Except for his body habitus and type 1 diabetes, there was nothing on history or physical exam that was suggestive of a secondary cause of hypertension. He had no proteinuria, microalbuminuria (early marker of diabetic kidney disease, which in this poorly-controlled patient is a real possibility), or signs of nephritis on urinalysis. Being ill in the hospital can increase your BP, but by 30-40 mmHg is unusual. Upon further questioning, the patient's father states, "Oh yeah I forgot, they used the blue cuff in the hospital," referring to the small adult cuff that would have barely fit around the patient's arm.
In children, BP measuring technique is critical. Using a BP cuff that is the wrong size for the patient is one of the more common reasons for pseudohypertension. Cuffs that are too large will underestimate the true BP and those that are too small with overestimate it. For assessing proper cuff size, the length of the BP cuff bladder should cover about 80% of the patient's arm circumference and the width should cover about 40% of the arm circumference. All blood pressure measurements should be obtained in an upper extremity, preferably in the right arm. Oscillometric devices can be used to screen for hypertension, but any elevated BPs should be confirmed with manual auscultation. And did you know, despite what Google images shows, the bell of the stethoscope should be used for auscultation?
For proper measurement, which is hard to do in kids (and why it's so important to have multiple BP measurements at different visits before confirming hypertension), the patient should ideally be sitting up straight, quiet, with feet on the ground. The arm should be supported by the examiner, with the upper arm at the level of the heart. BP variability simply from differences in body or arm position are surprising, and I included a table below of some examples (not comprehensive) of how improper technique can affect BP measurement.